In one of his last acts before leaving office, Virginia Gov. Mills E. Godwin gave assurance today that the state will keep its promise to release about $38 million in interstate highway funds to Metrorail construction.

That assurance came in a letter to U.S. Secretary of Transportation Brock Adams, who had written Godwin Jan. 3 to say that the federal government would approve no more contracts for construction of Interstate Rte. 66 between the Capital Beltway and Washington until Virginia applied for the fund transfer.

Godwin's letter ended an amiable note what some Metro officials had feared would become another confrontation over rail funding with Virginia officials who have resisted heavy state support of the system.

Godwin often has questioned the feasinility of Metrorail because of its high construction costs and expected operating deficits.

One high Metro official greeted word of the letter to Adams with a sigh of relief and said, "We were afraid the Adams letter would set off a reaction in Richmond."

Instead, Godwin's letter warmly thanked Adams for his support of 1-66 and emphasized that only the continuing legal challenges to the highway had been responsible for the slow transfer of interstate funds.

Godwin agreed late in 1976 to give the $38 million to Metro in exchange for approval of I-66 construction by former Secretary of Transportation William T. Coleman. He also agreed that the state would absorb another $35 million in Metro costs for rail-of-way construction in the I-66 median strip.

I-66 has been under construction for more than six months, but Godwin, as he said in his letter to Adams, had decided to hold on to the Interstate money until the last legal challenges to the road filed by citizens organizations were dismissed.

He told Adams, however, that he and Gov.-elect John N. Dalton have concluded that these suits may not be terminated "for some months" and therefore the Interstate funds will be released now.

Godwin's four-year term ends on Saturday, so the actual transfer of the $38 million will be left to Dalton.

Godwin alluded in his letter to Adams to a meeting between the two officials a week ago in which he assured Adams that the state fully intended to keep its transfer promise.

The $38 million was to have been used to build Virginia approaches to the now abandoned Three Sisters Bridge project over the Potomac between Rosslyn and Georgetown.

Godwin clearly agreed to the transfer only to win approval of I-66 rather than to help Metro.Nevertheless, it may be cited by others as symbolic of Virginia support of the Metro system.

Maryland and District of Columbia officials who have initiated large transfers of their interstate allocations to Metro have found themselves explaining to their constituents why Virginia has not done so.

Although the release of interstate funds for transit construction is commonly referred to as a "transfer," actually no highway money is used for rail construction. Virginia, in this instance, will simply yield its claim on the highway money, which reverts to the national interstate pool, and Congress will be asked to reappropriate an identical amount for mass transit.

In another development that holds the promise of accelerated aid for Metro from Virginia, Northern Virginia legislators and Metro officials agreed today on a plan to seek early release of $10 million approved for rail construction by the General Assembly last year.

A restriction placed on that appropration by the Assembly, in response to Godwin's skepticism of the project, requires the governor to approve a Metro financing plan before the money is released.

The Northern Virginians decided to submit Metro's interim plan for completion of 60 miles of the proposed 100-mile system to Dalton within the next few days. Sen. Wiley F. Mitchell (R-Alexandria) said in an interview that he believes it is likely that Dalton will consider the interim financing plan sufficient to justify release of the $10 million.