UNC Resources Inc., an Annapolis firm involved in telecommunications, aerospace and nuclear industries, moved yesterday to reduce the stake of its largest shareholder, but a Houston executive who holds another large block of stock still looms as a takeover threat.

UNC said it reached an agreement in principle with Chevron Corp. for the repurchase of Chevron's 8 million shares by UNC or other investors over the next three years, including an initial 2 million shares at a price of $12.50 a share. Chevron's shares now account for roughly 36 percent of UNC's 22 million shares outstanding.

But because the move initially will reduce the firm's shares outstanding to 20 million, it also will have the effect of swelling to about 22 percent the UNC holdings of Maxxam Group Inc., a New York holding company for Charles E. Hurwitz, an investor who has been involved in a number of takeover scraps.

Hurwitz, who has been circumspect about his intentions at UNC, did not return calls this week. However, he showed his continuing interest in the firm in recent months when he purchased another 1 percent of the stock, bringing his holdings to a little over 20 percent even before yesterday's action, according to a February filing with the Securities and Exchange Commission.

But the UNC board and management have made it clear that they want nothing to do with the Texan, refusing Hurwitz a seat on the board of directors and adopting a "poison pill" takeover defense last fall that greatly increases the acquisition cost for a corporate raider.

Vern Diedrick, UNC senior vice president and chief financial officer, said yesterday the company has had periodic conversations with Hurwitz, but nothing concrete has emerged.

"He has not pushed about the seat or anything. Obviously, we know he's there," he said. "I'm sure that he knows if he had an interest to sell, we'd have an interest certainly to discuss it."

Both Diedrick and stock analysts who follow UNC Resources said circumstances have changed dramatically since Hurwitz first started buying stock more than a year ago. For one thing, the stock's price has risen, closing yesterday at $12.25, or several dollars higher than the price at which they said Hurwitz originally bought many of his shares.

In addition, UNC Resources underwent a major restructuring over the course of 1985, including the purchase of three new companies and the hiring of several new senior executives. The company had previously shed its uranium mining and milling operations, which used to provide a large part of its business.

One major effect of this reorganization has been to dramatically reduce the amount of cash on hand for UNC -- from about $300 million to less than $20 million, according to Diedrick -- thus apparently reducing much of the allure for Hurwitz.

The deal with Chevron announced yesterday represents a further effort by UNC to gain flexibility in financial and strategic planning, Diedrick said.

Chevron had paid $100 million for its stake in UNC as part of a settlement the company reached with UNC in 1984 over a legal dispute between UNC and Gulf Oil, which was subsequently purchased by Chevron. But the West Coast oil company has been seeking to sell those shares to raise cash needed to pay off debt incurred by the Gulf transaction, a Chevron official said.

As part of the initial deal, Chevron agreed not to sell its shares without UNC permission, and the deal announced yesterday effectively extends the "standstill" until 1989 as long as UNC lives up to its bargain to buy back the stock or arrange for other investors to buy it, officials said.