Net income for American Broadcasting Cos. fell 19 percent last year and 29 percent in the fourth quarter, before it was acquired by Capital Cities Communications Inc. for $3.5 billion, an officer of the newly merged company said yesterday.

Anthony Herrling, manager of investor relations for Capital Cities-ABC Inc., confirmed part of a Wall Street Journal report that said the 1985 earnings and revenue figures were contained in a Securities and Exchange Commission filing on a debt offering by the company, formed in January.

ABC had earnings of $158.4 million last year, down from $195.3 million in 1984, and that revenue fell 11 percent to $3.31 billion from $3.71 billion, Herrling confirmed.

The fourth-quarter results were not disclosed specifically, but the Journal said that by subtracting the nine-month results from the full-year results, ABC apparently earned $37.1 million in the quarter, down from $52.4 million a year earlier, and revenue rose slightly from $932.9 million to $947 million.

Wall Street analysts have said they expected ABC's 1985 performance would reflect its last-place ranking in the three-network ratings race. They also have said ABC's stronger 1984 performance was distorted by heavy advertising for the Olympic games and national election coverage.

*Lorimar-Telepictures Corp., producers of such television programs as "Dallas" and "Knot's Landing," reported a sharp drop in earnings for the first six months of fiscal 1986.

For the period, the entertainment company's earnings dropped to $3.1 million (39 cents a share) compared with $23.3 million ($2.64) for the same period the year before.

The company said the drop had been anticipated because of Lorimar's increased television development and production, advertising investments and the disappointing domestic box-office performance of "Power," starring Richard Gere.

Merv Adelson, Lorimar-Telepictures' chairman and chief executive officer, said profit margins are typically reduced during growth periods. He added that last year's figures included revenues and operating income from the initial domestic syndication of "Dallas."

*Prudential Insurance Co. said yesterday its 1985 net income rose by 58 percent from the previous year, including a reversal of the net loss for its Prudential-Bache Securities Inc. unit.

Prudential also announced it had agreed to acquire an commercial insurance company based in Ohio in order to enter the commercial insurance market.

The Newark financial services company, which is the nation's biggest insurer, reported net income before policyholder dividends of $2.33 billion during 1985, compared with net income of $1.48 billion the previous year.

At a news conference in New York, Prudential Chairman Robert Beck attributed the strong gain in earnings to a 94 percent increase in sales, plus profitable results in nearly all of the 110-year-old company's divisions.

Net income for Prudential-Bache Securities rose to $43 million, compared with a $85 million loss during 1984. Beck said part of the 1984 loss was attributable to major capital expenditures and costs stemming from large staff changeovers.