Negotiations went into extra innings this morning in an effort to avert a strike by the Major League Players Association. And, early today it appeared the negotiators were rallying.

Federal mediator Kenneth Moffett said progress had been made and advised reporters to "stick around for another hour. There may be something at that time. They have been bargaining all afternoon and all evening."

Asked about the atmosphere in the negotiating room at the midtown hotel where the negotiators had been meeting since 10 o'clock last night, Moffett said, "It's good."

Asked if the players who had boarded planes last night in order to be at today's games had done the right thing, Moffett smiled wanly and said, "I think the players have gotten on the planes."

The prospect for a come-from-behind victory for the beleaguered netotiators came as a surprise to reporters who had been advised earlier in the day that it would take "a small miracle," in the words of Marvin Miller, executive director of the players union, to avert a strike.

Seemingly inexplicably, just minutes after Moffett's announcement, the members of the owners' negotiating committee left the hotel.

Meanwhile, travel plans for teams due to move from one city to another for weekend series -- customarily the most lucrative playing dates -- were scrambled reportedly with the cooperation of the players. None seemed anxious to go home for indefinite, unpaid furloughs until definite word comes that there will be no games today.

Miller and Ray Grebey, chief negotiator for the owners of the 26 clubs, met privately after talks had recessed in the afternoon. Then they returned, with federal mediators, for last-ditch bargaining.

After the talks resumed at a midtown hotel late in the evening, a hint of a ninth-inning rally developed. Some sources said there was an outside shot at a settlement, but cautioned against overoptimism in fending off what would be the sport's third strike since 1890.

The sources said that the issue of compensating teams for departed free-agent players, the sticking point since the beginning of negotiations months ago, was discussed but that no new proposals were made. The difference, it was said, was simply that now there was more give-and-take, less stonewalling:

After their Thursday night game in New York with the Mets, the Houston Astros boarded a bus for Philadelphia for a scheduled game tonight with the Philles. Mets player representative John Stearns told his teammates to call the players association in the morning for further instructions.

In Los Angeles, the Dodgers left for Chicago on a flight at 11:30 p.m. EDT. The flight was rescheduled several times, pending late developments in the talks. The first game to be affected if a strike is called will be in Chicago, the Cubs versus the Dodgers scheduled at 2:30 p.m. EDT.

As he returned to the hotel for the yesterday's third bargaining session, with midnight nearing, Miller had said, "There's nothing definitive at this time. We've been talking on and off here and at another hotel -- Mr. Grebey and I talking off the record, no mediators, no taking notes. We were having exploratory conversation like before the negotiations ever began."

Asked at that stage if the prospect of averting the nationwide strike had improved, Miller said, "At this point in time, I had no more assurance of an agreement than last time I talked to you."

That had been at 6 p.m. when the talks recessed for dinner. At that time, David Vaughn, the general counsel for the Federal Mediation & Concilation Service, said there was "no guarantee" there would be further face-to-face talks. He declined to say whether there were new proposals on the table or whether the issue of free-agent compensation had been discussed.

"There's not exactly joy in Mudville," Vaughn said.

The last strike began on April Fools' Day eight years ago and lasted 13 days. "In 1972, it was a little different," said Richard Moss, then legal counsel for the players association. "The only reason for a strike then was because the owners believed the players would never strike. There was not a real issue then."

Now there is a real issue: free-agent compensation, a matter of principle to both sides that has proved intractable.

If the roadblock in the negotiations had been a question of a salary increase, for example, there would have been more room for compromise: one side demands a 70-cent raise, the other offers 50 cents and they compromise on 60. In the 1972 strike, the central issue was the amount of money the owners had to put into the pension and health benefit fund. The owners offered $5 million; the players asked for $6.2 million; they finally settled on $5.9 million.

But finding a middle around on an issue that both parties consider an absolute is far more difficult. Some sources say that for this reason a strike could prove to be a long one. If the parties cease talking, the sources say, the positions on a matter of principle can become even more hardened.

The positions on free-agent compensation are these: the owners have insisted that compensation is necessary to provide competitive balance in the major leagues. Their last proposal offered a system whereby a club losing a "premium" free-agent (one selected by eight or more clubs and finishing in the top one-third in at-bats of the players at his position) would be able to select any player off the other club's roster other than the 15 the club chooses to protect.

For a player selected by eight or more clubs and between the top third and top half of the players at his position, the club would be able to protect 18 players.

The players association believes that this proposal would take the freedom out of free-agency. The right to sell themselves to the highest bidder once a contract is completed was won by the players in the 1976 basic agreement that expired Dec. 31. It is not a right they are prepared to give back.

Moreover, the players contend that the owners' proposal defines the top 50 percent of players as "premium." The object of the proposal, they say, is not competitive balance, but a ploy to drive down salaries among the middle-level players included in the formula, whose salaries are "where the real money is, said Marvin Miller, the executive director.

"For every $800,000 player there are 30 or 40 on down the line," Miller added.

"They (the owners) can't level with us, so they sit there and lie. They say they aren't aiming at anyone's salary but the proposal gives the lie so that," Miller said.

Sources say that a fundamental lack of trust between the parties has made good faith negotiations difficult.

Yesterday talks began at 11 a.m., an hour later than expected, because the owners were late. Asked if the thought this was a psychological ploy, Miller said, "No, just incompetence."

When the talks recessed for lunch at 12:40, National League President Charles Feeney said, "There's life, there's hope. That's all I can say."

Asked what had been accomplished, federal mediator Kenneth Moffett replied, "Zilch."

Asked about the tenor of the discussions, he said, "These people are really well-behaved. The only problem is that they are 180 degrees apart philosophically."

The issue of compensation was not even brought up during the morning session. They talked instead about muddy fields.