What was going on when I was born?

Economic growth slumped to a 1.9 percent annual rate in the July-September quarter, the Commerce Department reported today. It was the poorest performance since the current expansion began in late 1982. The revised calculation of the gross national product - the nation's total output of goods and services - was significantly lower than the 2.7 percent estimated a month ago and confirmed the view of an growing number of analysts that the country may already be in a ''growth recession.'' Such a recession is characterized not by an actual decline in output but by growth so skimpy that the economy cannot absorb increases in the labor force, and by a consequent rise in unemployment.

An experimental plan to reduce the costs and ticket prices of Broadway plays - a plan that has excited interest and curiosity in the theater industry - will go into effect Friday when the drama ''Dancing in the End Zone'' begins rehearsals. Morton Gottlieb, the play's producer, has won a series of unusual financial concessions from the artists, labor unions and theater owner that reduce the cost of the production. In return, Mr. Gottlieb has agreed to use only 499 seats in the 954-seat Ritz Theater, thus putting the Ritz in a special ''small theater'' category, and to keep his top ticket price below $30. The play opens Jan. 3. The plan, in effect, allows a play the cachet of playing in a Broadway theater while holding prices closer to the Off Broadway level. As a result of the concessions, Mr. Gottlieb said, it will cost $450,000 rather than $800,000 to produce ''Dancing in the End Zone,'' and the top ticket price will be $29.50, compared with the high of $37.50 for dramas on Broadway.

An article in Business Day yesterday about Monday's stock market trading misstated an earnings prediction by John Blair & Company. The forecast was for reduced earnings in the fourth quarter.

After a 10-year battle with the Securities and Exchange Commission, Robert E. Brennan, the president of First Jersey Securities Inc., signed a consent decree last night in which he and his brokerage firm accepted a permanent injunction not to bid for stock in a company at the same time he was selling it to the public. The 40-year-old executive, famous for nationwide television commercials in which he steps from a helicopter to deliver a personal message to investors, called the decree ''a great day for me and First Jersey. We've been fighting for 10 years to get this.''

The International Business Machines Corporation yesterday agreed to sell the military computer division of the Rolm Corporation within six months, in order to get Federal approval to acquire the rest of the company. The agreement came in a consent decree with the Justice Department. The move leaves intact the bulk of Rolm, the Silicon Valley company for which I.B.M. made a $1.25 billion offer seven weeks ago. Although Rolm started in 1969 as a military computer company, the military specification division now accounts for only one- eighth of the company's business. The company's main attraction for I.B.M. is believed to be the telecommunications operation that it began in 1975.

CBS Inc. said yesterday that it had agreed to buy the 12 consumer magazines of the Ziff-Davis Publishing Company for $362.5 million, believed to be the highest price ever paid for a group of magazines. The agreement concludes half of a keenly watched auction that began when Ziff-Davis put its stable of consumer and trade periodicals on the block early last month. No buyer has yet emerged for the company's 12 trade publications, which cover particular industries. Ziff had originally hoped to sell all 24 publications to one company. Although there had been speculation that CBS was pursuing the whole lot, CBS said yesterday that it had submitted no offer for the business group.

T HE proliferation of margarine brands and types, which began in the mid-1970's with the introduction of margarine ''spreads,'' has reached the point where a shopper faces an almost overwhelming choice: there is hard margarine in sticks, soft margarine in tubs or liquid margarine in bottles; margarine with or without salt and reduced in calories; margarine made from corn, soybean, safflower, cottonseed or sunflower oil. The newest variation is a blend that contains a surprise ingredient - butter. Siert Riepma, president of the National Association of Margarine Manufacturers, a trade association, says the desire for a larger share of the market is responsible for the incredible variety. ''Everybody is trying to find some angle that will appeal to consumers,'' he said. ''They put butter in it. They put it in a two-pound tub, in a three-pound tub.'' The consumption figures are impressive. More than 2.5 billion pounds of margarine were spread on toast, baked into cakes and cooked with vegetables in the

G.N.P. growth slumped to a 1.9 percent annual rate in the third quarter, the lowest level since the recovery began in late 1982. The rate was well below the 2.7 percent growth estimated by the Commerce Department a month ago, and gave support to the view that the economy may be growing so little that unemployment will rise. The Commerce Department also said after-tax corporate profits fell 7.3 percent from the second quarter. (Page A1.) The Treasury finished drafting ''a new tax system,'' an Assistant Secretary in the department said. According to lobbyists, the plan would reduce or abolish the difference between tax rates on capital gains and other income and would adjust capital gains for inflation. The official said Treasury Secretary Regan had approved the draft, to be delivered to President Reagan by Dec. 1. (A1.)

Richard Brown, a hard-nosed parking garage owner, decided that he was going to be charitable this Thanksgiving - and he didn't care who tried to stop him. Business has been good to Mr. Brown this year, and he set out two weeks ago ''to give something back to this city'' - turkeys. He would give 70 frozen turkeys to the needy. He wearily recalled his attempts to find city officials in Manhattan who would take his turkeys. He said that wherever he called he was put on hold, transferred several times and finally told that they weren't interested.

A table in Business Day on Nov. 12 with an article about American oil refiners misstated the refining capacity of Standard Oil (Ohio). The company has current capacity of about 456,000 barrels a day; it has contracted to buy, but has not yet taken over, an additional 198,500 barrels.

When consumer spending surged last Christmas, the Crompton Company, America's oldest weaver, took it as a sign of good times to come and rejoiced. After three years of losses, the company assumed that consumers were in the mood to buy more clothes and that it would therefore sell more fabric. But Crompton was wrong: Orders poured in last fall, then stopped. Like the rest of the industry, Crompton had hoped Washington would restrict imports of fabric and apparel - but it did not. And with the stronger dollar making foreign goods even cheaper, Crompton sadly concluded that it could no longer compete.

''I am absolutely not being coy about it. I have an intention and that is my intention; I have a desire and that is my desire.''