It is one giant energy company -- Mobil Oil -- against one medium-sized energy company -- Virginia Electric and Power. But the giant says it is on the defensive.

The issue is a high-tension power transmission line that Vepco wants to build right through the middle of what will be the northern half of the new town of Reston, which is being developed by Mobil's subsidiary, Reston Land Corp.

"They (Vepco ) have a lot more power than us -- they can use eminent domain to condemn our land," says Reston Land's Vice President for Design and Engineering Richard P. Bonar.

Bonar claims the power line would have a "devastating" effect on the 10 to 12 planned clusters of houses near which it would pass. "They're trying to tell me that property values won't be affected," he said. "But that's simply not true."

Bonar said Reston Land wants the line built along another path farther away. But Gregory Wroniewicz, director of engineering services for Vepco, said the route preferred by Reston Land is less desirable because it would disturb the Sugarland Run Stream Valley and also affect a nearby Fairfax County park.

"I don't see any major problem" with the Vepco-preferred route, Wroniewicz said. "I think we could work with Reston people so that the line would not adversely affect the town."

He said the transmission line's 230,000-volt, 100-foot-high pylons would rust to a deep brown color that would be less prominent than the silver color of traditional lines.

Reston -- a new town built from scratch on thousands of acres of virgin land -- was supposed to be so well-planned that problems such as this never would arise.

But somewhere a glitch developed, possibly in the Fairfax County governmental bureaucracy. When Vepco first sent its proposal to the county, Wroniewicz said the Office of Comprehensive Planning replied that the Vepco-favored route was among several acceptable choices. Armed with the county's tacit approval, Vepco began to move ahead.

"Those knuckleheads in the county, they don't follow things," said Calvin F. Larson, a member of Reston Community Association's planning and zoning committee. "They do not understand that there has been a master plan for this area in existence since 1962," he said.

"The first I heard about it was on Friday," said Fairfax Planning Director Theodore J. Wessel. "I don't know what position we took. We're looking into it."

Fairfax Supervisor Martha V. Pennino (D-Centreville), who represents Reston, said she opposes any line that would cut across residential development.But she says the county planning office's action "will make it more difficult" to oppose the route favored by Vepco.

Pennino said she will ask Vepco to put the line under ground. But Wroniewicz said an underground line is impractical because it would cost four to six times the price of an overhead line, which he put at $300,000 to $400,000 a mile. The Reston line would extend 3 1/2 miles.