Integrated oil companies should be required to divest themselves of their pipeline operations, the assistant attorney general for antitrust, Donald I. Baker, said yesterday.

In his last appearance on Capitol Hill before leaving his post to return to Cornell University, Baker told the Senate Antitrust and Monopoly Subcommittee that the division is evaluating whether divestiture can be accomplished through court action.

In response to a question from subcommittee chairman Edward M. Kennedy (D-Mass.), Baker said he thought the case for divestiture was "quite compelling."

Because of the division's extensive investigations into pipeline operations and because of the President's commitment to antitrust enforcement and competition in energy, division sources yesterday said yesterday there is no reason to believe that those who follow Baker in leadership positions will have a different view.

He explained that the integrated company can do this by limiting others' access to its pipeline operations and by reducing the amount of its own oil sent through the pipelines, thus forcing delivery of some crude oil or refined product by more costly transportation alternatives and causing prices to rise.

"Baker also told the subcommittee the division recently has begun a "focused" investigation into possible price fixing by intrastate natural gas producers and pipelines. Informed sources say the division has sent 30 civil investigative demands - requests for information with the force of subpoenas - to companies in Texas, Louisiana, Oklahoma and New Mexico as part of its probe.