Frederick S. Pierce, who guided ABC to the heights in the late 1970s and had a key role in the $3.5 billion merger between Capital Cities Communications Inc. and ABC Inc. this past year, resigned yesterday.

Only a week ago today he officially became vice chairman of Capital Cities/ABC Inc., as well as chairman and chief executive officer of ABC, when the merger was concluded.

Yesterday it was announced that Pierce, 52, will remain on the board of Capital Cities/ABC Inc. and will become a nonexclusive consultant to the communications giant.

Heralding a major reorganization of ABC, John B. Sias, who has been president of the Capital Cities Publishing Division, was named president of the newly formed ABC Division, responsible for the ABC Television Network, Entertainment, News and Sports divisions.

Reportedly Pierce's positions will not be filled.

Sias joined Capital Cities in 1971 as head of Fairchild Publications after a 15-year broadcasting career with Metromedia and Westinghouse. He is a member of the Capital Cities board.

Yesterday, Capital Cities/ABC Inc. Chairman Thomas Murphy said Pierce's decision to resign, which he made known on Monday, had come "as a surprise."

Murphy told a reporter yesterday that "Fred decided in his own best interests that he had run everything at ABC long enough and had successfully and safely steered the ship into the merger.

"We tried everything to keep him. And don't forget he stays with us as a real consultant. We had talks three or four weeks back in which we discussed what he might want to do. I can understand why he made the decision for emotional reasons."

Pierce met with senior staffers at ABC yesterday for about an hour to explain his decision.

He told them that three or four weeks ago he had begun to have second thoughts about his long-term role with the new company. Following a skiing vacation at Snow Mass, Colo. ("10 fantastic sunny days that gave me lots of time to think"), he returned to New York last weekend.

Monday night he made his decision to resign, notifying Murphy immediately.

At his own suggestion Pierce will accompany Murphy and Daniel B. Burke, president and chief operating officer of Capital Cities/ABC Inc., to Los Angeles next week to introduce them to the Hollywood creative community.

Pierce, who will remain in his offices at ABC until Feb. 1, will move to Capital Cities headquarters across town in Manhattan later that month as a paid management consultant.

Pierce joined fledgling ABC in 1956 as a research analyst, rising to president of ABC Television in 1974.

In his five-year tenure in that post, the network -- never before considered a major contender against CBS and NBC except in sports programming -- became No. 1 in primetime, daytime and early morning programming, while establishing competitive programming in news, children's shows and late night.

He was named executive vice president of ABC Inc. in 1979 and in 1983 succeeded Elton Rule as president and chief operating officer.

Several years ago, Pierce made the first informal contacts with Murphy about the long-range possibilities -- should Federal Communications Commission rules eventually relax -- of a merger between their two companies. He later played a major role on behalf of ABC Chairman Leonard Goldenson in the subsequent discussions with Capital Cities that climaxed in the announcement of the merger last summer.

Recently, ABC experienced a slide in its primetime ratings and there had been some criticism of Pierce, as the top executive, for not moving more quickly to reverse the slump.

Several months ago he reorganized the ABC Broadcast Group and his longtime ally Anthony Thomopoulos left the company. Meanwhile, with the prospect of cost-conscious Capital Cities moving in this year, he ordered severe budget cuts that eventually slashed ABC rolls by 350 jobs. Yesterday Pierce told a reporter that "I figured after 30 years I'd pretty much accomplished what I'd set out to do and it's a time for new horizons."

He said that he and Murphy had held "very open and frank discussions on an entirely friendly basis" for several weeks before he made his decision. "He wanted me to stay, but I told him the time had come to do other things. It's better to do it right now and not wait until later."

As a consultant, Pierce said, "I will give him as much help as I can in the transition. In addition, I hope to help ABC Entertainment President Brandon Stoddard with his more immediate programming plans.

"As to future plans, I have no idea. For 30 years I've worked for ABC. I really haven't given it any thought, but I'll be around broadcasting, some way or another."

Pierce does not leave ABC empty-handed. His contract was to run through 1989 and the negotiated settlement on his remaining salary gave him $1.75 million.

In addition, the recent merger reportedly raised the value of Pierce's ABC stock holdings to more than $4 million.

One More Thing

Production of "Amerika," a multimillion-dollar ABC mini-series about a Soviet takeover of the United States, has been postponed following a Soviet threat to jeopardize the network's Moscow news operations, ABC said yesterday.

"There was a discussion with our correspondent in Moscow about the unhappiness with which they viewed the program," said David Burke, vice president of ABC News.

On Wednesday, ABC Entertainment Division President Brandon Stoddard said the network was reconsidering its plans for the mini-series because of budgetary problems.

But Burke said the Soviets took a dim view of the program and expressed their displeasure to ABC Moscow Bureau Chief Walter Rodgers in a Dec. 15 meeting at the Soviet Foreign Ministry.

The Soviet officials "made it clear to Walt how unfortunate it would be if the relations that ABC News had in Moscow diminished" due to the program, Burke said.