In an abrupt about-face that surprised some of his colleagues, Senate Majority Leader Robert C. Byrd (D-W.VA.) decided this week that he wanted to sit on the Foreign Relations Committee, and then changed his mind.

A letter stating Byrd's interest in a seat on the committee went to members of the Democratic Steering Committee -- which makes committee assignments -- early Monday morning.

But before the close of business the same day, steering committee members received a second Byrd Letter that said, "On further consideration, I have decided not to seek a position on the Foreign Relations Committee."

The quick change surprised colleagues because it contradicted the image that Byrd has long cultivated as a deliberate and decisive politician. "It just doesn't figure," one said.

Byrd's withdrawal from consideration from Foreign Relations was a blow to the Carter administration, and to Sen. Frank Church (D-i/daho), who is in line this month to become the committee's new chairman. Both had hoped Byrd would join Foreign Relations to lend his legislative skills and his stature as majority leader to the committee and its work.

Asked about his audden change of heart yeaterday, Byrd said it was brought on by the belief that he should not seek an exception to Senate rules merely to accommodate his own desire to be on Foreign Relations.

The rule precludes any senator from serving on more than one of the Senatehs five major committees (Foreign Relations, Appropriations, Commerce, Armed Services and Finance). Byrd is the third-ranking member of the Appropriations Committee, a position that has enabled him to direct considerable federal aid to his constituents in West Virginia.

Originally, Byrd said, he thought the Democratic Steering Committee would waive the rule for the Majority leader, "but the more I thought about it the more I thought I wouldn't like the idea of asking for an exception."

Several colleagues and Senate aides said yesterday that this explanation seemed less than fully satisfactory. "It doesn't track," one said.

Another noted that the first Byrd letter announcing his intention to seek a seat on Foreign Relations, though dated Jan. 3, was delivered Jan. 8. "I hear the letters were sitting on his desk while he thought about it," this source said.

Byrd noted that Senate Minortiy Leader Howard H. Baker Jr. (R-Tenn.). is already a member of Foreign Relations, and that Byrd's predecessor as majority leader, Mike Mansfield, also served on the committee.

But in his second letter, Byrd said, "My heavy duties as majority leader would preclude my spending as much time as I would like attending committee hearings and meetings."

One Senate aide speculated yesterday that the urge to serve on Foreign Relations came from "Byrd the candidate for president," whereas the decision to drop the idea came from "Byrd the West Virginia senator."

Another source who has worked closely with the majority leader said the explanation might be simpler. "Byrd isn't as methodical and well organized as a lot of people think," he said.

After Byrd's withdrawal from consideration for a seat on Foreign Relations, informed sources said the likely new Democratic members would be Sen. Edmund S. Muskie of Maine and Sen. Paul Tsongas of Massachusetts, a freshman.