The sudden incapacitation of President Reagan yesterday revealed considerable confusion in the White House over the line of authority in the even of a national crisis.

Everyone agreed on the legal line of succession in the event of death; vice president, speaker of the House, president pro tempore of the Senate, secretary of state and so on. And everyone agreed that if a military emergency arose while Reagan was, for example, under sedation, George Bush would take command if available.

Beyond that, it seemed to depend on who was speaking. Shortly after 4 p.m., Secretary of State Alexander M. Haig Jr., his voice quavering with emotion, appeared in the White House briefing room.

Yes, "crisis management is in effect," he told reporters in response to a question that included the term involved in controversy last week. "Constitutionally, you have the president, the vice president and the secretary of state in that order.

" . . . As of now, I am in control here, in the White House, pending return of the vice president. . . ."

But at 9 p.m., presidential spokesman Larry Speakes was saying something else. Secretary of Defense Caspar W. Weinberger was, after the vivce president, to take command in the event of a crisis, Speakes said.

Weinberger, not Haig, was third in command. This line of succession follows the military chain of command, in which the president is the nation's commander-in-chief, automatically followed by the vice president and then the defense secretary.

A call to a spokesman for the vice president produced yet another reaction. He had not heard specifically of anybody being third in command. "If there is a crises," Bush press secretary Peter Teeley said, "the vice president is the head of crisis management and would be responsible."

The confusion was understandable, in light of the rapidity of the day's events. But David Gergen, deputy to the White House chief of stafff, acknowledged later that officials had not researched the law and the Constitution on the subject and that the arrangements described by Speakes were "informal. . . . I think there will come a time when we can give you more information on this."

As for Haig, White House officials stressed that this time, despite suggestions by reporters, they were pleased with Haig's performance earlier in the afternoon when he took charge in the difficult moments just after the shooting. But it was not clear last night who asked him to do so.

In the Constitution, the 25th Amendment, ratified in 1967, provides a framework for the temporary replacement of a disabled president.

If he is able, the president need only send a letter to the president pro tempore of the Senate and the House speaker informing them that he cannot carry out the office's duties.

The vice president immediately becomes acting president until the president notifies the president pro tempore and speaker that he is ready to resume his duties.

If the president is unwilling or unable to write the letter, the situation becomes more complicated and potentially expolsive. Under the Constitution, the vice president and a majority of the Cabinet would officially have to declare the president "uanble to discharge the powers and duties of his office."

If the president insisted that he could continue to serve, the entire matter would have to be resolved by Congress within 21 days. For the vice president to continue in office, two-thirds of both houses would have to agree that the president was incapable of serving.

Otherwise, the president would stay in office.

To some extent, the late Speaker of the House John W. McCormack can be tanked for the relatively smooth procedure embodied in the 25th Amendment.

After John F. Kennedy's assassination, Lyndon B. Johnson served without a vice president since there was then no procedure for appointment of a new vice president. McCormack, then 72 and looking every bit his age, was next in line for the presidency.

Johnson had suffered a heart attack in 1955 and the thought of a President McCormack helped bring about enactment and ratification of the amendment.

It was and is still criticized, however, for failing to require a medical judgment on disability, among other things.

In the event the vice president, Bush, cannot or will not serve, the line of succession established by Congress is the speaker of the House, Thomas P. (Tip) O'Neill Jr. (D-Mass.), the president pro tempore of the Senate, Sen. Strom Thurmand (R-S.C.), and the secretary of state, Haig.

The 25th Amendment also established the procedure for selecting a new vice president. It was used by Richard M. Nixon when Spiro T. Agnew resigned and by Gerald R. Ford, when he replaced Nixon as president.

The new vice president is nominated by the president and confirmed by a majority vote of both houses of the Congress.

Because of its recent addition to the Constitution, the disability provisions have never been tested. But the country was thrown into heated debate about it three times inthe past: when James A. Garfield was assissinated in 1881, when Woodrow Wilson was critically ill 1919 and 1920 and when Dwight D. Eisenhower suffered a heart attack in 1955.