William J. Small, president of NBC News for the past 2 1/2 years, abruptly resigned yesterday morning, citing "differences in management style" for his surprise action.

The resignation was effective immediately. The network said a replacement would be named next week. Speculation immediately centered on "NBC Nightly News" anchor John Chancellor as Small's successor. Chancellor, a former director of the Voice of America, is due to step down as anchor in April in favor of Tom Brokaw and Roger Mudd.

Another leading candidate reportedly is NBC News senior executive producer (and former NBC News president) Reuven Frank.

Others rumored yesterday to be on the list were ABC News senior vice president (and former NBC News president) Richard Wald and CBS vice president for hard news Ed Fouhy.

In a telephone conversation yesterday, Small declined to discuss the reasons for his departure or elaborate on the "differences."

"I've been in broadcasting for 20 years and if I may say so, I think it's a pretty distinguished record," Small said. "I don't want to comment--it's their store."

As for his personal plans, Small said, "I'll worry about it next week."

Small, 55, began his broadcasting career in 1956 in Louisville and in 1962 was named Washington bureau chief for CBS News. By vigorously promoting a daily Washington presence on the "Evening News" and shrewd selection of both on-air and production talent, Small made an impact on TV news gathering that is still reflected at all three networks.

In 1974 he was made CBS News senior vice president, director of news, and in 1978 was named vice president, Washington, for CBS Inc. A year later he jumped to NBC News.

Small was hired away from CBS by then-NBC chairman Fred Silverman, shortly after William Leonard had been named president of the CBS News division. At the time, Small's defection from CBS after 17 years was widely regarded as an expression of his disappointment at having been passed over for the CBS News presidency.

Small quickly recruited other CBS news executives, producers and on-air talents to fill posts at NBC News, an infusion of talent that was widely resented at NBC, as was Small's abrasive style, long familiar--and understood--at CBS, but which won him few friends in his new job.

Perhaps his biggest coup was the acquisition of Roger Mudd, himself disappointed last year when Dan Rather was named to succeed Walter Cronkite as anchor of CBS Evening News. Marvin and Bernard Kalb and Betsy Aaron are others who followed Small to CBS, as did top producers like Leslie Midgley and Sylvia Westerman.

Last summer, however, Silverman was ousted as the top echelons of both NBC and parent company RCA underwent major reorganizations. Periodically since then, broadcasting circles have buzzed with rumors that Small's tenure at the network was in jeopardy.

Those rumors focused on the rise of Robert Mulholland to the post of NBC president under new chairman Grant Tinker, a post that put Mulholland in charge of the News Division. Mulholland was a former NBC News executive vice president who reportedly shared with his former News colleagues resentment over the "superior" attitude of the former CBS staffers toward the NBC news organization.

Although he has been circumspect regarding Small in his public statements, Mulholland reportedly has lent a sympathetic ear to NBC News employes regarding the ostensible morale problem created by the "outsiders" from CBS.

Last summer it was rumored in New York that Mulholland wanted Small fired but that the incoming RCA chairman, Thornton Bradshaw, in the wake of the Silverman firing and the departure of 38-year NBC veteran David Brinkley to ABC News, ordered a stop to the public "blood-letting" at NBC to give the new network management a chance to settle in.

Brinkley himself would never comment on his reason for leaving, but insiders blamed Small's handling of the veteran and his "NBC Magazine" staff for the departure.

Those matters aside, however, Small is considered one of the best TV newsmen in the business. Most assumed that NBC would stay with him since there were few news executives at any network capable of replacing him.

Small's three-year contract with NBC was to expire in September and there had been no direct hints from management that he would not be routinely renewed this summer.

In response to a question from a reporter, NBC chairman Tinker said earlier this year that Small could have the job as news president as long as he wanted.

Last month Tinker told a reporter, "Bill Small is . . . a damn good professional head of news. In truth, he is two things. He's that stand-up comic we saw with the affiliates who love him because he is a pro. But I guess he's a hard guy to work for. He's just kind of an autocratic guy, I guess, or at least he seems that way and doesn't go out of his way to stroke everybody. I don't have any misgivings about what I see Bill Small to be in terms of the head of news."

A top broadcasting executive and longtime friend of Small's said yesterday: "Bill is a very decent human being. But I've got to admit, sometimes he seems to defy you to like him."

Small revealed his decision at his regular morning staff meeting yesterday. Subsequently, he issued a statement in which he said:

"I am grateful to hundreds of NBC News staffers who have worked with me in the last 2 1/2 years to help NBC News continue as one of the world's great news organizations. I am also grateful to the officials of NBC and the many affiliates who have supported the efforts of our news organization. I am proud of the achievements of so many superb journalists in the News Division, and am especially pleased, as I leave, that NBC News is strong and very competitive by every standard of good journalism."

Actually, during Small's tenure at NBC, the "Nightly News" remained competitive in the battle with "ABC World News Tonight" for second place behind "CBS Evening News." However, NBC failed, as did ABC, to mount a sustained challenge to CBS last spring when the departure of Cronkite caused a drop in the "Evening News" ratings.

Nor did NBC's "Today" show succeed in overtaking ABC's sprightly "Good Morning America" on a regular basis during Small's reign.

Within the industry, however, it is generally believed that Small's changes at NBC News did generate some needed vitality in a news organization that had grown complacent with its solid second-place ranking.