A Prince George's County group announced plans this week for a $10 million conversion of the aging Equestrian Center in Upper Marlboro to an all-weather, 3,000-seat indoor arena with a prime restaurant.

The plans to renovate the center and restore some of its grandeur from bygone days were unveiled at a news conference Monday in front of about two dozen commercial and real estate developers and county and state officials.

Dorothy Troutman, president of the group formed in 1984 to market the facility, the Friends of the Equestrian Center, said the renovated arena would provide quality horse shows and races. In addition, she said, it would have a tremendous impact on the local and state economies by attracting premier equestrian events. "The economic spinoffs this arena would create are so great that it would be to the state's disadvantage not to do it," she said.

Bill Chambers, general manager of the equestrian center, said the Maryland General Assembly is expected to consider a bill this session that would provide $5 million to help fund the changes.

County Executive Parris Glendening, who has tried to attract development to Prince George's, said the county would match a $5 million appropriation from the state.

"It is one of the additional amenities that are expected in the county, and I will ensure funding from the county if the state agrees to it," Glendening said. Besides Glendening's support, the project has received backing from several local politicians, including County Council Vice Chairman Anthony Cicoria, and state Sen. Frank J. Komenda (D) and Del. Joseph R. Vallario (D). But some horse enthusiasts such as Frank DeFrancis, owner of the Laurel and Bowie race tracks and Pimlico race track in Baltimore, are reserving comment on the project.

The local business community has also endorsed the project, although many said that they are more interested in economic spinoffs than in the equestrian events.

Gary Lockman, developer of the upscale Lake Arbor residential development in Mitchellville, said that besides the economic benefits, he sees the arena as a way to strengthen the state's growing polo-playing population. "Something like this is of the proper dimensions for polo and I support it wholeheartedly," said Lockman, an avid polo player.

The arena would have 56,000 square feet of floor space, a lounge area and banquet rooms, administrative offices, a snack bar, concession and a covered warm-up arena. The multipurpose facility could also be used for sporting events, concerts and trade shows, the promoters said.

The history of the equestrian center dates back to 1905, when horses raced on the half-mile track before throngs of spectators. In the midcentury, the track was hailed as among the best in the state.

Known as the Marlborough Race Track, it was the place to be for horse lovers, state dignitaries and the wealthy. The business thrived until the late 1960s when it ran into financial problems.

In 1971 the Marlborough Race Track lost its racing rights and closed its doors. "It used to be quite a place with the racing and the betting. Not too many people even know it's there today," Troutman said. "It's rather sad." For nine years after it closed, the center fell into disrepair. A fire gutted the grandstand area and it became a vandals' haven. "At that point it was just an eyesore until the county came in and took over in 1980," Chambers said.

The center was rejuvenated under the county's direction, and in 1981 the Maryland-National Capital Park and Planning Commission assumed control of the facility. It has had minimal renovation to make it suitable for horse shows and other events.

It is now leased by horse groups from New Jersey to the Carolinas for shows. Horse trainers around the state use the track to train their horses for races at other tracks.

Alvin Turner, developer of the Presidential Corporate Center and vice president of the Friends of the Equestrian Center, said the arena would not be for the wealthy only. "I'm not a horse person and I've only ridden a horse twice and that nearly scared me to death, but I will go to an event," he said.

Turner said he also views the project as a way to preserve what agriculture still exists in the state. He said the dying tobacco industry and other cash crops have pushed the horse and feed industry into the forefront of farming.