The number of the day's first decision was announced, but so softly that few people heard it. An Associated Press reporter grabbed the papers, glanced over the case title, gulped loudly and sprinted past four rows of colleagues in the Supreme Court press room. In those few seconds, the assembly's collective blood pressure went up.

As he dashed toward the phone in the adjacent office, he yelled "It's Zenith," and the collective blood pressure went back down. Zenith was an important case; but it was not as big as Bakke, the one they've all been waiting for in recent weeks. "I almost had a heart attack, the way he took off," muttered one reporter. "God there's got to be a better way." It was the first of five such false alarms that day.

In the massive, grand Supreme Court Building, a siege is underway. A media vigil. Waiting for Bakke, a countdown that grows more tense as the court nears the term's end, has the press corps jittery each morning the court sits. This week the court is meeting four days and is expected to meet at least three days next week.

On those mornings, NBC and ABC have full camera crews outside. Usually 25 reporters cover the proceedings of the court. This week the number in the bare, strictly functional newsroom swelled to nearly 50, with some news organizations assigning an extra person. Couriers are on standby. Elaborate relay systems have been mapped out. Phones are scarce, but manners good-natured.

"Bakke, Bakke," echoed Carl Stern, NBC's veteran court reporter, leaving one unproductive vigil. "Personally I don't recall any other case that has had this excitement, this anticipation, this long. Undoubtedly the closest thing was the Nixon tapes. There are enormous pressures to get this story just right. It obivously hangs over your head and you are constantly reciting to yourself what you think you might say. I go over the probable stories in the shower, in taxis. Mind you, I will forget it all. But it is consuming."

The case of the Regents of the University of California v. Allan Bakke has become one of the most important civil rights cases since the historic school desegregation decision in 1954. More briefs were filed with this case than any ever at the Supremen Court.

Bakke, a 38-year-old engineer, sued the University of California Medical School at Davis after he was denied admission in 1973 and 1974. He charged discrimination because 16 places in the class of 100 were set aside for minority students in a special admissions program. His grievance is that several of those students had lower test scores than his.

The decision, according to most legal experts, will have far-reaching effects on the affirmative-action programs in education and employment, and related issues, such as compulsory government percentages for minority contractors. Most civil rights lawyers believe a decision in favor of Bakke will be a step backward.

In the press room most of the reporters have several versions of their story prepared. "We usually have one back-breaker a year and this is it," said Tom Stewart of Reuter. "I plan to run in here, sit down and try to resist the temptation to call something in, for the sake of calling it in. I am going to take three minutes to read the decision - that's a long time - but this is very important."

The winning network will probably be the one whose correspondent does not slip on the marble floor or crash into a gaggle of tourists. Since May 1, NBC has had a two-man crew with a portable microwave transmitter set up on the lawn outside the Methodist Building. The long cables from their equipment have been attached to the antenna on top of the building, giving a tightrope act air to their stake out location. Inside the court NBC has two correspondents, radio and television, an artist and a field producer. In the studio on Nebraska Avenue they have a full control room on alert.

Because the Bakke decision will probably be announced during the West Coast broadcast of "Good Morning America," that show has a producer at the court. Other ABC personnel include the radio and television correspondents and a reporter inside, and a three-person crew on the court steps. This week the technical crew has been reading "Notes to Myself" by Hugh Prather and playing backgammon. Stationed by the elm tree on the Capitol grounds are three ABC people, who will feed the video to the network.

CBS doesn't have a crew outside but Fred Graham, the legal correspondent, has a live microphone at his desk. "The editor at the other end has been reading 'War and Peace.' But the day it happens I will do a radio alert and the television will pick up a version of what I say for their bulletin." said Graham. Also this week WETA started sending an associate producer to court, just to rush back to the office with a copy of the decision so it can be distributed in New York, Washington and San Francisco for specials that will be televised from those cities.

Meanwhile at 9 a.m. the telephone starts ringing in the press office. Barrett McGurn, the court's chief press officer and a former foreign correspondent for The New York Herald Tribune, patiently tells all the callers to wait until 10:30 a.m. "Most of the calls are about Bakke. Each year there's a big case, the death penalty cases, the tapes of President Nixon. But we all have to wait."

McGurn has even added a new precaution. One day last week Tim O'Brien the ABC correspondent, stood in front of McGurn's desk, where all the day's decisions are piled, face down. "But if you look carefully you can make out the numbers and letters.So I counted. Now this is reading upside down and backwards and I yelled out 'Snail Darter.' McGurn got so mad that now all the piles are covered with a plain sheet of paper. So this vigil has stimulated some innovations."