July 30, 1985

EVIDENCE is building that development and growth of the brain go on into old age. It was once thought that the brain was fixed by late childhood, according to innate genetic design. As long ago as 1911, however, Santiago Ramon y Cajal, a pioneering neurobiologist, proposed that ''cerebral exercise'' could benefit the brain. But a scientific consensus that the brain continues to bloom if properly stimulated by an enriched environment was long in coming. ''Over the last decade, neuroscientists have become impressed by the degree to which the structure and chemistry of the brain is affected by experience,'' said Floyd Bloom, director of the division of neuroscience and endocrinology at the Scripps Clinic and Research Foundation in La Jolla, Calif. The new research seeks to provide a more detailed understanding of that phenomenon.

President Reagan refused today to support the two central elements of the latest Senate budget plan: tax increases, including an oil import fee, and alternate-year adjustments of Social Security benefits to account for inflation. The announcement of the President's decision, awaited since Thursday, put an end to the Republican-led effort in the Senate to put together a plan to cut projected budget deficits by more than $300 billion over the next three years. ''Any chance for this year of getting a real, significant, reliable deficit reduction package is gone,'' said Pete V. Domenici, the chairman of the Senate Budget Committee. New Talks Due Today Directing the blame at the White House, he added: ''In this case there is not a will to address the huge deficit.''

An article July 18 about CBS public-service announcements to help find missing children cited broadcasters' efforts incompletely. Since last year, ''Good Morning America'' on ABC has broadcast information on missing children.

Sales at the nation's department stores and auto dealerships have suffered in recent months because of consumers like Tammy and Daniel Nelson. Last year and the year before, when things were going well at Mr. Nelson's job, the Chicago couple purchased several costly items, including a refrigerator and an air-conditioner. This summer, however, the Nelsons have cut their purchases because the metal-crafting company where Mr. Nelson works has reduced his schedule to four days a week. ''Business is slow and we're not rushing out to buy many things,'' Mr. Nelson commented recently.

President Reagan and the Soviet leader, Mikhail S. Gorbachev, each offered new proposals on the testing of nuclear weapons today. The White House announced that Mr. Reagan had invited the Soviet Union to send observers to witness an underground nuclear test at a Nevada test site, an Administration official said. Hours later, Mr. Gorbachev announced through the press agency Tass and on national television that the Soviet Union would impose a five-month moratorium on nuclear weapons tests. He said the moratorium would start on Aug. 6, the 40th anniversary of the United States bombing of Hiroshima, and would continue beyond Jan. 1 if the United States imposed a similar moratorium in the meantime. Meeting in Helsinki The two announcements came as Secretary of State George P. Shultz and the new Soviet Foreign Minister, Eduard A. Shevardnadze, arrived in Finland for a three-day meeting marking the 10th anniversary of the Helsinki accords on European security and cooperation. [Page A6.] Mr. Shultz told reporters traveling with him to Helsinki that the United States did not believe that ''it is in our interest to stop our testing program under these circumstances.'' He also expressed doubts about the sincerity of the Soviet moratorium.