Gov. Gerald L. Baliles, expressing concern about possible terrorism at National and Dulles International airports, said today he had directed the Virginia National Guard to develop a "readiness plan" to combat terrorism.

Baliles said he was concerned about the two airports because of their proximity to the nation's capital and said he wanted the 9,700 members of the guard to have a plan to respond to attacks.

The governor's announcement came as he reappointed Maj. Gen. John G. Castles as state adjutant general, head of the Virginia guard. Baliles said he had told the general to plan for possible "hostile actions against civilian and military facilities, involving transportation and shipping."

Castles, 61, said the guard likely would be used as "a backup force" to respond after an attack, with the primary responsibility for preventing attacks resting with the FBI, CIA and federal forces.

Baliles said that, unlike the governors of Maine and Massachusetts, he had "not seen any evidence" that the guard should not train in Central America. He said the guard had trained there and in Europe in the past and that federal officials should decide where the guard should train.

Castles said federal officials had told him that the air guard's base at Richmond International Airport, which houses 27 fighter jets, is "one of the top targets" of possible terrorism because of its location.

Also today, Baliles named his third study commission. This one, headed by Lt. Gov. L. Douglas Wilder, will look into the affect of federal budget cuts on state programs. Previously, Baliles named citizens groups to advise him on transportation and education.

The new 28-member commission includes eight persons from the Washington area, including Stuart E. Eizenstat, who was President Carter's domestic policy adviser. Eizenstat, who lives in Chevy Chase, was chosen, according to a governor's aide, because he recently participated in a similar study for Maryland.

Northern Virginians named were Alexandria City Manager Vola Lawson; Harris N. Miller of McLean, a public relations consultant; Leonard Shapiro of McLean, a developer; Ruth S. Hanft of Alexandria, a health policy research consultant; James F. Fort of Alexandria, a lobbyist; Steve Farber, a former executive director of the National Governors Association, and Terry Smith, who has been a lobbyist for Florida and Kansas in Washington.