Negotiators for the National Football League and the striking NFL Players Association met in secrecy today for 15 hours and were to meet all night with mediator Sam Kagel as the NFL's first regular-season strike entered its 23rd day.

Kagel imposed a news blackout for the time the mediation effort is in progress, but there were no tangible signs of progress in the dispute, which has been at a virtual stalemate since last February.

However, representatives of both sides said they were prepared to bargain around the clock to resolve the stalemate and Kagel was said to have informed the parties to be prepared for four days to a week of continuous bargaining.

Asked this afternoon if he would call for 24-hour bargaining, Kagel said, "If we need to, we will."

Although Kagel, a private attorney and veteran mediator and arbitrator from San Francisco, has forbidden either side to make any public comments while the mediation is going on, he did agree to brief the news media once daily in the late afternoon on how the talks were progressing. Today he said only, "The parties have had extended meetings with me. Both last night and, of course, beginning this morning and this afternoon. And we expect to go into the evening.

"And the specific statement is that the parties' proposals and counterproposals on all issues are being reviewed and discussed in depth and this process will continue."

Kagel, a gnome-like, bearded, pipe-smoking man of 73 who plays basketball to keep in shape, met initially today with the NFLPA's full negotiating committee. That included Ed Garvey, executive director of the union; Gene Upshaw of the Los Angeles Raiders, its president; lawyers Dick Berthelsen and Joseph A. Yablonski; Mike Duberstein, economist; three player representatives -- Tom Condon of the Kansas City Chiefs, Mark Murphy of the Washington Redskins and Stan White of the Detroit Lions, and Leonard Lindquist of Minneapolis, the senior partner in Garvey's old law firm.

After a discussion of the union's position, Kagel then met separately with the management side: Jack Donlan, the executive director of the NFL Management Council; Sargent Karch, general counsel; Vincent Lombardi, assistant executive director; Terry Bledsoe, assistant general manager of the New York Giants, and Steve Guttman, secretary-treasurer of the New York Jets. After exploring the management position, Kagel met again with Donlan, Garvey, Karch and Upshaw to map out an agenda for the mediating process.

There was no indication whether the discussions would move first to the main stumbling block or attempt to tackle side issues first. The primary issue is the players' demand that a trust fund be set up to pay them on a seniority-based scale with performance incentive bonuses -- and management's opposition to that concept.

The mediation process, the first full-scale attempt to resolve the dispute across the bargaining table since talks broke off in Washington Oct. 2, is being held at the Hunt Valley Inn in this Baltimore suburb.