A House oversight subcommittee has asked American Telephone & Telegraph Co. and MCI Communications Corp. for a wide variety of documents in connection with a major investigation of the telecommunications business.

Most of the documents sought by the House Commerce oversight and investigation subcommittee concern the $2.7 billion antitrust suit brought by MCI against AT&T in Chicago.

Disclosure of the subcommittee's investigation comes at a critical juncture in the MCI lawsuit and in House Commerce Committee negotiations concerning legislation that would revamp federal regulation of the telecommunications industry and the structure of the Bell System.

The MCI case is in recess until Monday, when AT&T begins its case. Negotiations over the legislation, which was written by Rep. Lionel Van Deerlin (D-Calif.), chairman of the House communications subcommittee, are the subject of talks among key members of the House Commerce Committee. The Van Deerlin subcommittee passed the bill two months ago.

"I have been concerned about AT&T and questions of monopoly operations in many areas," said Rep. Bob Eckhardt, chairman of the House oversight subcommittee.

Eckhardt's letter was sent simultaneously on March 20 to Charles Brown, chairman of AT&T, and William McGowan, chairman of MCI. It asks for a broad set of documents concerning competition in the industry since 1970.

Eckhardt primarily is seeking documents that have been released from a court protective order in the MCI case. Already hundreds of pages of AT&T and MCI documents have been released in connection with the case.

An AT&T spokesman, Pic Wagner, said that the company was "surprised at the request because of the enormity of the proposed vast amount of information which seemed to be involved."

Wagner said AT&T representatives met with members of the subcommittee staff to "see if they really want billions of pieces of paper as the letter seemed to imply."

AT&T officials are attempting to identify precisely which documents have been requested by the Eckhardt panel.

MCI already has delivered to the subcommittee a staff a series of documents related to the Eckhardt inquiry. "The more light that is shed on this industry, the more people will realize that the industry is archaic from the point of view of the economy as a whole," McGowan said. "The more people understand, the better."

Eckhardt is undecided about his view of Van Deerlin's bill. He said that his probe in no way is designed to interfere with that legislation. Eckhardt didn't discuss the probe with Van Deerlin before the letters were sent.