April 7, 1985

The Secretary General of the United Nations, Javier Perez de Cuellar, announced today that, in a change of plans, he would visit Iran and Iraq during his Persian Gulf tour to try to help end the war between the two countries. Mr. Perez de Cuellar, who made the announcement in Doha, Qatar, will fly to Teheran on Sunday and to Baghdad later in the week. In a visit here five days ago, he said he would not visit the two warring countries during his tour because Iran had refused to give assurances that it was willing to discuss a comprehensive settlement of the four-and-a-half-year- old war. 'A Positive Development' The Secretary General now feels that his conditions have been met, according to an aide reached by telephone in Doha.

. . . THE HEAVENS AND THE EARTH; A Political History of the Space Age. By Walter A. McDougall. Illustrated. 555 pp. New York: Basic Books. $25.95. A GRADE-SCHOOL youngster once wrote to T. Keith Glennan, the administrator of NASA, saying she wanted to know ''everything about space.'' In the margin of her letter Mr. Glennan scribbled ''So do I.'' So too does Walter McDougall, a historian at the University of California, Berkeley. ''. . . The Heavens and the Earth'' is just as cosmic and all-encompassing as its title suggests.

THERE is no other firm in the world quite like Merrill Lynch & Company. It is the giant of the brokerage industry, from New York to London to Tokyo. In the United States, Merrill's brokerage offices have been known for decades in nearly every hamlet in the land - about 90 percent of all Americans live within 20 miles of a Merrill branch. More recently, the firm has elbowed its way into the upper echelons of investment banking, becoming a major force in the world of high-stakes corporate finance. But the giant is faltering. Its 1984 financial results were dismal. Merrill earned $95 million, compared with $230 million the year before - on revenues in the same $6 billion range. Its pre-tax profit margin was a slim 1.2 percent, and its 4.8 percent return on equity was far below the industry norm.

To say that a dream came true with the back-to-back Broadway openings of Peter Nichols's ''Joe Egg'' and Neil Simon's ''Biloxi Blues'' is not to indulge in idle rhetoric. The dream that came true was an actual dream - the one in which the dreamer runs into some old friends and discovers, to his amazement, that the familiar faces and personalities have suddenly been altered almost beyond recognition. So it is in the two productions that have lifted a moribund Broadway season from its ides of March (''The Octette Bridge Club,'' ''The Loves of Anatol,'' ''Requiem for a Heavyweight'') to an Easter-eve near- resurrection. In ''Joe Egg,'' two comic actors of New York theatergoers' long acquaintance, Jim Dale and Stockard Channing, have turned themselves inside out to provide performances that are as shattering as they are funny. In ''Biloxi Blues,'' an even older friend, Mr. Simon, has written his most heartfelt comedy - with the result that we can never again pigeonhole a writer who had become the most popular American playwright in part by becoming the most cautious.

-Nimeiry lost power in the Sudan yesterday the way he won it 16 years ago - by a military coup. It was in large measure the price for yielding to the United States and the International Monetary Fund, which had pressed him to impose austerity on a people already burdened by poverty. In Washington last week, he reaped a reward - $67 million in economic assistance that the Reagan Administration had been withholding and another 225,000 tons of grain for some 6 million starving people.