President Carter, in a two-hour meeting with U.N. Secretary General Kurt Waldheim, last night rejected a proposal that a U.N. commission investigate the regime of the deposed shah of Iran and reiterated American insistence that the United Nations impose economic sanctions against Iran.

Carter's position, which amounts to a demand that the United Nations make good on an earlier resolution calling for economic sanctions against Iran if the 50 American hostages were not freed by today, was relayed to reporters after the meeting by White House press secretary Judy Powell.

"The president reiterated the position of the United States that the United Nations begin without delay the process of implementing the United Nations Security Council resolution of Dec. 31," Powell said.

Powell also indicated that Carter had rejected a suggestion, voiced earlier yesterday by Waldheim during a television interview, that a special U.N. commission be created to investigate deposed Shah Mohammad Reza Pahlavi as part of a "package deal" aimed at obtaining the freedom of the hostages being held at the U.S. Embassy in Tehran.

"The president also made clear the previously oft-expressed position that international forums could be made available for the airing of Iranian grievances, but not before the release of the hostages," he said. " . . . That is the single issue.All other concerns are not relevant."

Although today was the deadline set in the Dec. 31 resolution for release of the hostages, officials said last night they do not expect a resolution that would implement economic sanctions against Iran to be introduced in the U.N. Security Council until later this week.

Moreover, although Carter last night insisted on the imposition of sanctions, there remains considerable doubt over how tough a sanctions measure -- or whether any measure at all -- can be pushed through the Security Council.

Waldheim, who returned to New York Friday after a three-day visit to Iran, flew to Washington yesterday afternoon to report to Carter on his mission. He met in the Oval Office of the White House with the president, Secretary of State Cyrus R. Vance, national security affairs adviser Zbigniew Brzezinski and Donald McHenry, U.S. ambassador to the United Nations.

The Dec. 31 resolution on sanctions also called on Waldheim to travel to Iran in an attempt to obtain the release of the hostages who have been held by militant Iranians in the embassy since Nov. 4. The militants, with the backing of Iran's revolutionary leader, Ayatollah Ruhollah Khomeini, are demanding that the deposed shah be returned to Iran to stand trial for his "crimes" against the Iranian people before the hostages are released.

The resolution called for the imposition of economic sanctions against Iran if Waldheim's mission should fail, which it did, but did not specify what those sanctions would be.

The resolution was adopted by an 11-to-0 Security Council vote, with the Soviet Union among those abstaining.

Among the questions that cloud the future of a new resolution imposing sanctions is the possibility of a Soviet veto, particularly in light of deteriorating Soviet - American relations brought on by the Soviet invasion of Afghanistan.

The possibility of a special U.N. commission to investigate the shah as part of an arrangement to free the hostages was raised earlier yesterday by Waldheim on the television program "Issues and Answers" (ABC, WJLA).

"There are two aspects," he said. "We want the hostages -- not only the Americans, the international community wants the hostages. The Iranians want the shah and want an inquiry into the violations of human rights by the previous regime. So we have to work out some sort of package deal which makes it possible that we get the hostages, but we also have to accept something which satisfies the Iranians."

Waldheim said he discussed the idea of a U.N. commission with Iranian Foreign Minister Sadegh Ghotbzadeh, and Iran's Revolutionary Council and that all agreed it "would be a possibility." Although earlier in the interview Waldheim described the militant students who hold the hostages as out of the control of Ghotbzadeh, the Revolutionary Council and possibly even Khomeini, he said the suggestion "will hopefully have the approval of the students."

Waldheim will report to the Security Council today on his mission to Iran, and yesterday, before meeting with Carter, said that he would raise the U.N. commission as a "possibility" but not a recommendation.

In the interview, Waldheim also voiced skepticism that the imposition of economic sanctions on Iran will have any effect on that country's leaders.

"Quite frankly, I am doubtful whether it will solve the problem," he said. "I raised the matter with Iranians . . . And they say: 'We couldn't care less. It wouldn't do harm to us.'"

Waldheim refused to say whether he was allowed to see U.S. Charge d'Affaires Bruce Laingen, who is being held at Iran's foreign ministry. The militants at the embassy have demanded that Laingen be turned over to them for questioning.

The U.N. secretary general was not allowed to see the embassy hostages, and yesterday he provided a gloomy assessment of the chances of winning their release soon, saying this may take weeks or even months.

He said the hostages are not in "immediate danger," but that they would be endangered if the U.S. resorted to military action.