The Securities and Exchange Commission, in its massive report on the October stock market collapse, has questioned the performance of some of the specialists who play a pivotal role in maintaining orderly trading on the floor of the nation's stock exchanges.

The SEC delivered a mixed verdict on the role played by specialists on Oct. 19 and Oct. 20, when the stock market was experiencing its greatest convulsion ever.

"While most specialists performed well under the circumstances on the morning of Oct. 19, there were several instances of questionable specialist activity," the report said. Questionable actions continued for several days, according to the SEC.

Specialists are so named because they specialize in trading specific stocks, assigned to them by the exchanges. Their job is to match orders from buyers and sellers on the exchange floor or to act as buyers or sellers when it is necessary to balance the flow of orders.

The SEC report called on the New York Stock Exchange and the American Stock Exchange to investigate the trading that took place on their floors on Oct. 19 and 20 and to keep a tighter rein on their specialists.

One of the key themes of the SEC report was that the some of the specialists appeared to be selling more stock than they were buying at times when they should have been doing just the opposite to offset the flood of selling orders from institutional and individual investors.

Richard Torrenzano, vice president of the New York Stock Exchange, said the presidential task force headed by Nicholas F. Brady and the SEC staff had "invented" a new system for measuring specialist performance. An NYSE measurement system, he said, gives the specialists "high marks" for their performance during the collapse.

He acknowledged that there were "isolated incidents" where problems developed and said that two NYSE specialists, who were trading the shares of Gould Inc. and J.P. Morgan Co., had resigned from those stocks because of events during the week of Oct. 19. The stocks were reassigned to other specialists, he said.

Investigations into the performance of other specialists are under way, Torrenzano said.

The American Stock Exchange said it planned to review the activity of its specialists in all of its 900 common stocks.

The SEC examined trading in nine NYSE stocks and nine Amex stocks, chosen because of heavy selling, trading halts and wide price swings.

Among the Amex stocks looked at by the SEC were The Washington Post Co. and Giant Food Inc. Trading in The Post's stock is also under study by the Amex performance review committee, according to Ivers W. Riley, senior executive vice president at the Amex. Trading in Giant Food will be reviewed, he said.

The Post drew attention because trading in its stock was halted for 2 1/2 days -- Oct. 19, Oct. 20 and part of Oct. 21.

As the SEC related it, Post stock closed at $221, down $14, on FridayOct. 16. The specialist in Post stock was a heavy buyer and at the end of that day and was "long" -- or owned -- 16,413 shares.

On Monday, Oct. 19, the stock did not open for trading because the specialist faced a heavy imbalance of sell orders. He had orders to sell 2,400 shares at the "market" and "limit" orders to sell 2,100 shares at between $177 and $240.

Market orders are orders to be executed at the current price. Limit orders are orders to sell at specific prices.

To test the market in Post stock, the specialist indicated he would be willing to buy at $180, but later in the day, as the market dropped, lowered that bid price to $150 a share.

He also offered a tentative selling price of $210 and later, $200 a share. But the stock did not open for trading until Oct. 21 at 12:11 p.m., the report said. It opened at $194, down $27 from its previous close, on a volume of 11,400 shares.

"The stock rose to $204 by 3:20 p.m. {on Oct. 21} during which time the specialist was almost exclusively a seller. The stock dropped to $199 on trades of 5,200 shares, of which the specialist bought 100 shares," the report said.

The SEC offered no opinion on the specialist's performance in this case.

Richard D. Simmons, president of the The Washington Post Co., said, "This study represents a continuing evaluation of both the stock market and the specialist system. Pending whatever is the final conclusion, I don't think it is appropriate to comment."

David B. Sykes, senior vice president for finance at Giant Food, said there was normally little contact between the Amex specialist in Giant stock and the company.

He noted that the specialist, Damm, Frank & Co., had gone under during the collapse and was acquired by Bear, Stearns & Co., which now handles the stock.

"We sell groceries," he said. "It's their job to make a market in the stock."