More than 1 millin square feet of space was leased in office buildings here in May and June, the Julien J. Studley real estate firm reported. Nearly half of the space was in downtown buildings. In surburban Maryland, 339,000 square feet of office space was leased.

In both cases the totals were greater than those of a year for those months. In Virginia, 279,000 square feet of office space was leased in May and June. This was about 100,000 feet less than a year earlier.

Recent leasing transactions:

The American College of Obstretricians and Gynecologists signed a lease for 50,000 square feet in the new Capital Gallery complex being built by Boston Properties at 600 Maryland Ave. SW. The college will move its national headquarters from Chicago next summer.Leggat, McCall & Werner is the agent for Capital Gallery. Coldwell Banker was cooperating broker in the lease.

The Charles E. Smith Companies reported two major leases in Three Skyline Place at Baileys Crossroads. Two floors were taken by Tractor Electronic Systems and one floor by Tandem Computers Inc. The first tenants will move in next month.

The Winston Co. leased 80,000 square feet to Singer Co. in the Montgomery Industrial Park in Silver Spring. The space will be used for engineering and assembly of computer simulators.

Julien J. Studely Inc. arranged a least of 9,300 square feet to McClure-Lundberg Associates Inc., a minority firm specializing in management and consulting, at 1110 Vemont Ave. NW. The firm also will take 4,600 feet for printing facilities at 1146 19th St. Nw.

H.G. Smithy Co. has signed an exclusive leasing and management contract for headquarters for United Services Life Insurance Co. being constructed at 950 N. Glebe Rd., Arlington, for 1982 occupancy.

The architecutural firm of Cohen and Haft Holtz Kerxton Associates announced that ground has been broken for a second Springhill Lake office building. It will have 138,000 square feet of space. The first building was leased within five months of completion.

Boston Prperties has announced that the National Council on Aging has leased 91,500 square feet in the Capital Gallery office complex at 600 Maryland Ave. SW. Occupancy is scheduled next summer, when the NCA will move from 18th and L streets NW. Coldwell Banker handled the leasing.

Centennial Computer Corp., a major adapter of computers to specialized funcitions, is expanding its area operations by signing a $1.1 million, long-term contract to take the 19,000-square-foot sixth floor of the Executive Building in suburban Rockville. In August, Centennial will begin to consolidate its Texas and California offices in the new space. Arthur Rubloff Co. handled the negotiations.

SCS Egineers, a firm with major offices in Reston and Long Beach, Calif., has leased 18,000 square feet for fall occupancy in a new CENTEC building in Reston. Wellborn Management handles the leasing in the building chosen by SCS for its Reston Expansion.

Resource Consultants Inc. has leased 18,900 square feet in a new building at 8200 Greenboro Dr., Tysons Corner, according to Julien J. Studley Inc.

The 38,000-square-foot Spring Palza South office building being completed at 700 Roeder Rd., Silver Spring, has been fully preleased by the Bethesda office of H. g. sMithy co. Occupancy is scheduled this fall.

The law firm of Houger, Garvey, Schubert, Adams and Barer has signed a 10-year, $2 million contract for 10,200 square feet in the Flour Mill Plaza complex on the Georgetown waterfront. This project of Weissberg Development Corp. includes new and rehabilitated office space and a luxury condominum apartment.

The John Hancock Mutual Life Insurance Co. has relocated from downtown Washington to 4,500 square feet in the new Air Rights III office building in mid-Bethesda. H.G. Smithy Co. completed the leasing arrangements for the tenant.

Meanwhile, the Metropolitan Area Transit Authority has announced it is soliciting lease and development proposals for a 150,000-square-foot land and air rights site at its Bethesda Station at Wisconsin Avenue and Edgemoor Lane. The station is due to be in operation late in 1983. Site proposals are due Sept. 30.