CIA Director William J. Casey bought and sold millions of dollars in stocks and other securities last summer as the stock market was beginning a major advance, according to his financial disclosure statement.

In a 26-day period from Aug. 26 to Sept. 20, Casey bought from $1.5 million to more than $2.2 million in stocks and other securities, particularly in electronics and drug companies.

Casey's heavy trading coincided with a broad advance in the Dow Jones Industrial Average, which stood at 892 on Aug. 26, peaked at 1,232 in early May and was at 1,202 yesterday.

While it is virtually impossible to gauge the profit or loss of Casey's extensive transactions during 1982, an examination of three of the stocks in which he made sizable investments shows that they have increased in value by 16 to 38 percent since August.

If Casey still owns the three stocks, he apparently would have a paper profit of between $104,000 and $250,000.

Casey has declined to place his financial holdings in a blind trust voluntarily, as many other senior government officials have done, including President Reagan, Vice President Bush and Treasury Secretary Donald T. Regan.

Instead, CIA spokesman Dale Peterson said, Deputy Director John McMahon and general counsel Stanley Sporkin review all of Casey's financial transactions to determine whether there is any conflict between his private holdings and his public role as the nation's chief intelligence officer.

If the two find such a conflict, Peterson said, they will advise Casey to disqualify himself from decision-making on matters that could affect the value of his holdings.

Peterson called the arrangement "the director's means of assuring there would be no conflict of interest," but he would not disclose whether McMahon and Sporkin had found conflicts in Casey's financial dealings. He said the information would be available upon request to congressional oversight committees.

The new policy was adopted last year after Casey's previous financial disclosure statement showed that he had sold more than $600,000 in oil company stocks during 1981 as a glut developed in world oil markets.

"The system that was set up last year is functioning and will remain the way it is," Peterson said. "There is no reason under the law why Casey has to put his finances into a blind trust." He added that the investment decisions generally are made by Casey's private financial adviser, Richard Cheswick.

Congressional staff members said they are not aware of any findings by the CIA officials reviewing Casey's transactions. "While some of our members don't like the way Casey does business, he is free to buy and sell anything he wants," an official with the Senate Select Committee on Intelligence said.

Several of the drug companies in which Casey bought substantial amounts of stock--including Abbott Laboratories, Bristol-Myers, Merck and Co., Johnson & Johnson and Pfizer Inc.--have foreign subsidiaries, including several in Central America. Abbott, for example, has subsidiaries in El Salvador, Argentina, Chile, Venezuela, Guatemala and Mexico.

Casey also bought considerable stock in electronics firms such as Digital Equipment Corp. and MCI Communications Corp.

Casey apparently financed his purchases last summer by selling off large amounts of Treasury bills and oil-company stocks, although he continued to buy lesser amounts of Treasury securities throughout the year. For all of 1982, he bought from $3.8 million to more than $7.8 million worth of stocks, securities, bonds and Treasury bills, while selling holdings worth from $3.7 million to more than $7.3 million.

While Casey may have lost money on some stocks, three of his larger investments have done well. Since Aug. 26, for example, Abbott stock rose from 33 3/8 to 43 3/8 a share, an increase of 30 percent; Digital increased from 80 1/2 to 111 1/8, or 38 percent, and Merck rose from 74 to 86, or 16 percent.

Peterson said he could not discuss the substance of Casey's investments.

Casey and his wife reported outside income of $579,000 to more than $1.03 million during 1982, along with assets of $5.17 million to more than $9.38 million. Precise figures are not available because the Ethics in Government Act requires officials to report income and assets only in broad ranges, with a top category of $250,000 and up.

In other 1982 financial disclosure statements made public in recent days:

* Secretary of State George P. Shultz reported outside family income of $1.27 million to more than $1.46 million. This includes $558,706 from his former employer, the Bechtel Group, from January, 1982, until he took office last July, and $19,775 in directors' fees from General Motors Corp. Shultz also reported assets of $1.8 million to more than $2.8 million.

* Labor Secretary Raymond J. Donovan reported income of $541,000 to more than $612,000. More than $100,000 came from Schiavone Holding Corp. of New Jersey. Donovan owns about 40 percent of the firm, which includes assets of a construction company and several subsidiaries.

* Treasury Secretary Regan reported income of $146,000 to more than $246,000, and assets of $965,000 to $1.66 million, which does not include the value of a blind trust, which was not revealed.

* Agriculture Secretary John R. Block reported assets of $3 million to more than $3.7 million, but also listed liabilities of $6.8 million to more than $9.6 million, a substantial increase from 1981. An Agriculture spokesman said that Block had to borrow more money to cover operating expenses on his Illinois farm, and that he also made improvements, bought new machinery and built a hog barn.

* Commerce Secretary Malcolm Baldrige reported income of $232,000 to $367,000, including $108,813 in pension benefits from Scovill Inc., the Connecticut manufacturing firm he formerly headed, and assets of $1.3 million to $1.7 million, not including a blind trust.

* Defense Secretary Caspar W. Weinberger reported income of $240,000 to more than $415,000 and assets of $1.52 million to more than $1.87 million.

* Trade representative William E. Brock reported income of $330,000 to $787,000 and assets of $3.39 million to more than $6.97 million, not including a blind trust.