Lockheed Martin Corp., the nation's largest defense contractor, said yesterday that it will spin off its interest in a construction materials business to its shareholders.

The company said it will swap shares in Martin Marietta Materials, a producer of construction aggregates, to shareholders in return for shares in Lockheed Martin, which is based in Bethesda. Lockheed Martin owns 81 percent, or more than 37 million shares, in Martin Marietta Materials. Details of the transaction, including how many Lockheed Martin shares will be swapped for each share of Martin Marietta Materials, won't be announced until the Securities and Exchange Commission approves the proposed offering.

The construction materials business was one of the earliest building blocks in what now is Lockheed Martin Corp. In 1961, when Glenn L. Martin Co. merged with American Marietta Co., a materials company that became part of Martin Marietta. Back then the materials business also included aluminum, a key aircraft material.

In recent years, however, the materials business has been peripheral to the company that resulted from the 1995 merger of Lockheed Corp. and Martin Marietta. "There isn't any doubt that these are two companies in extremely different businesses," industry analyst Wolfgang H. Demisch of BT Securities said.

"The record of the materials business is extremely good, and the returns have been exceptional," he said. "But it's also true that I don't know anyone who buys Lockheed Martin stock for the sake of the success of the materials business." In fact, he said, that success hasn't won the recognition it might because it is blurred into the other business.

By executing a trade for the Martin Marietta Materials shares instead of selling them to investors, Lockheed avoids potential capital gains taxes, analysts said. Lockheed could later resell its own shares to reduce the debt it incurred to help pay for its $9 billion acquisition of Loral Corp. earlier this year.

Martin Marietta Materials, with revenue of more than $660 million, has its headquarters in Raleigh, N.C. In addition to construction aggregates, the company is a leading producer of magnesia-based products.