First Chicago Corp., the nation's 10th-largest bank holding company, yesterday unexpectedly disclosed heavy third-quarter losses resulting from bad loans, under pressure from bank regulators who recently reviewed the company's loan portfolio.

First Chicago, the parent of First National Bank of Chicago, said loan losses in the third quarter of approximately $279 million will result in a net loss of between $70 million and $74 million for the period. The troublesome loans were split between energy and agriculture loans in the United States, and shipping and construction loans abroad.

"We expect the provision for loan losses in the fourth and subsequent quarters to return to a level more comparable to that of the second quarter, and we anticipate earnings for the fourth quarter and 1985 to recover accordingly," First Chicago Chairman Barry F. Sullivan said. The company reported a $45.7 million loan loss in the second quarter.

"We are confident that a provision of this magnitude is a one-time event," Sullivan said.

On Wall Street, First Chicago stock, which traded as high 27 during the past year, fell 3 1/4 to 20 and was the second most actively traded issue on the New York Stock Exchange. The First Chicago announcement, coming after Tuesday's lowering of an analyst's forecast for Citicorp, caused many of the money-center bank stocks to fall in early trading yesterday, although some recovered ground.

First Chicago Chairman Sullivan said the Comptroller of the Currency's examiners were "tough-minded" when they visited the bank recently, and that influenced First Chicago's decision to write off the loans now and to announce the losses prior to the regular quarterly earnings report on Oct. 12.

Sullivan said the company is not experiencing any liquidity problems and said that, even though the company's stock price fell sharply, the announcement was treated as a "nonevent" in the money markets, where the company continued to have no problem raising capital. Sullivan confirmed rumors that First Chicago has been extending the maturity of its certificates of deposits recently to bolster its cash position in anticipation of yesterday's announcement.

Analysts were surprised by First Chicago's announcement and said they were looking forward eagerly to Sullivan's meeting with securities analysts in New York this afternoon, when they would have a chance to discuss the situation with him. They said they believe the latest disclosure of problem loans would not have significant repercussions at other banks.

"It is clear the bank examiners are taking a tougher look at loans in light of Continental and pressure from Congress," said Mark Biderman, an analyst with Oppenheimer & Co.

"For sure, there are going to be some more credit problems in the industry but this bank is not another Penn Square," said James Wooden, an analyst with Merrill Lynch.

Sullivan "said this announcement is more like a bad inning in a baseball game. I'm sure he would rather have been out at Wrigley Field watching the Cubs yesterday," Wooden said.

Analysts were not the only ones surprised by yesterday's announcement. Sullivan said he also was surprised by the deterioration of the loans and indicated that, among the loans to troubled industries that led to the losses, only energy loans remained a significant portion of the company's loan portfolio following yesterday's write-offs.

He emphasized that the remaining energy loans on the company's books were overwhelmingly to major corporations.

First Chicago is the third major domestic bank holding company to report significant losses this year. Continental Illinois was rescued by the federal government in July in a $4.5 billion bailout, and Financial Corp. of America, parent of the nation's largest savings and loan association, has faced a liquidity crunch since the company was forced by the Securities and Exchange Commission to restate its second-quarter $31.3 million profit as a $107.5 million loss, following a dispute over the company's accounting techniques.

First Chicago expects to "enter into an agreement with the Comptroller of the Currency" under which it will revise its capital plan to provide for improvement in its ratio of primary capital to assets and to strengthen its management of "criticized assets," the company said.

The $70 million to $74 million quarterly loss in loan losses announced yesterday by the $40.5-billion-asset bank holding company are equivalent to third-quarter losses of between $1.75 and $1.85 a share. The company earned $1.95 a share during the first half of 1984.