President-elect Ronald Reagan has selected six men for key jobs in his administration, informed sources said yesterday, but he has not yet made a final choice for the posts of secretary of state and secretary of the treasury.

The six nominations that now appear "quite certain," these sources said, are Caspar W. Weinberger to be secretary of defense, William French Smith to be attorney general, Sen. Richard S. Schweiker (R-Pa.) to be secretary of health and human services, Drew Lewis to be secretary of transportation, William J. Casey to be director of the Central Intelligence Agency, and Rep. David Stockman (R-Mich.) as director of the Office of Management and Budget.

Retired general Alexander M. Haig continues to be the leading contender for secretary of state, sources said, but the selection is not final. Press reports that Walter B. Wriston, chairman of Citicorp, will be Reagan's secretary of the treasury are premature, these sources said. Both Reagan and Wriston are still "hesitant" about his selection, according to one source close to the president-elect.

Alan Greenspan, chief of the Council of Economic Advisers in the Ford administration, is still a possibility for the treasury, some sources said, and there may also be others.

Yesterday's round of name-playing brought no big surprises, nor any certain information about when the president-elect would reveal his final selections. According to sources in California, Reagan hopes to introduce his Cabinet choices personally, probably in small groups, but it is not known when this might happen. Next week in Washington seemed possible.

The president-elect has managed to maintain a substantial degree of secrecy about the way he is going about the selection of Cabinet members, even if he has not been able to keep all the names of the candidates confidential. Sources close to Reagan have said the president-elect will talk personally to his selections, but none of the candidates for Cabinet jobs has admitted publicly to direct contact with Reagan, by phone or otherwise.

Haig said in Hartford, Conn., yesterday that he had not had any word from Reagan. Haig said he had "no idea when or whether" Reagan will offer him a job. He called the State Department job attractive, but declined to say that he would accept it.

The emerging Reagan Cabinet includes a heavy dose of political rewards.Drew Lewis, the apparent choice for secretary of transportation, is a Pennsylvania businessman whom Reagan installed as his representative this fall at the Republican National Committee. The chairman of that committee, Bill Brock, reportedly is under consideration for secretary of commerce, deputy secretary of state or ambassador to the United Nations.

Schweiker, Reagan's reported choice for health and human services, was Reagan's designated running mate in his unsuccessful 1976 bid for the Republican nomination for president. Schweiker voluntarily gave up his Senate seat this year to work for Reagan's election.

Casey, the likely new director of CIA, was Reagan's campaign manager.

Well-informed sources said yesterday that despite the heavy speculation in the news media, the secretaryships of commerce, energy, housing and urban development, interior and labor are still "really open."

Agriculture, while still undecided, is likely to go to John R. Block of Illinois, a successful farmer and that state's top agriculture official, these sources said. The job has become a hotly contested one. Clayton Yeutter, president of the Chicago Mercantile Exchange, is still a possibility for agriculture, the sources said.

Reagan apparently is interested in finding a woman, a black and perhaps a Democrat to serve in his Cabinet, but if that is the case, those members remain to be chosen.

According to informed sources, Reagan is continuing to discuss possible appointees with key advisers by telephone. Most of his principal associates are now in Washington, where the president-elect will be next week.