John and Ginny Hanlon are proud of the rustic three-story house that John, a home builder, designed and built in a secluded, wooded area of northern Montgomery County eight years ago. And until recently they didn't worry about the fact that their new home adjoined a Potomac Electric Power Company electric transmission line.

Today, though, the Hanlons are quite worried, Pepco is considering use of the line's right-of-way to build a much more high-powered transmission system - the massive size and voltage of which raise serious health and land-use questions. Construction of the [WORD ILLEGIBLE] transmission line might even result in the loss of the Hanlons' dream home.

The Hanlons didn't know it when they first learned about the new transmission line plans, but their concern now links them to citizens around the nation who are battling the construction of high-voltage power lines.

As electric companies everywhere build transmission systems to carry increasingly higher voltages, an army of towering metal creatures dragging mile after mile of supercharged wire has started marching across the American countryside.

They are leaving in the wake questions about possible biological and environmental harm and complaints from angry homeowners. In places like New York, Minnesota, North Dakota and California, communities that once might have accepted meekly the appearances of new towers and transmission lines have now started asking, "Why us?"

In Maryland "why?" is being asked about the last segments of a 500,000-volt transmission line being built in a 240-mile circle around Washington in Maryland and Virginia. Pepco, which is building the line in a joint project with the Baltimore Gas and Electric Co. and the Virginia Electric and Power Co., says it is needed to meet this area's growing energy needs and strengthen the system's ability to give and receive power from other parts of the country. The old line looping this area carries only 230,000 volts.

The company has been working on the 500,000-volt loop - with 135-foot towers carrying the transmission lines - since the late 1960s. Over half is now completed.

The segment that would run through the area where the Hanlons dwell - a 10-mile stretch of rolling countryside interpersed with housing developments and farms, running from Brighton in upper Montgomery County to High Ridge in lower Howard County - is one of the last segments still awaiting government approval.

Most of the line has gone forward with relatively little controversy, but Pepco has run into unexpectedly strong opposition over the Brighton-High Ridge section. During the past year, Pepco has proposed no less than five different routes to the Maryland Public Service Commission. Everywhere it turns, Pecpo meets someone who wants the line put somewhere else.

The southernmost route is opposed by residents of Montgomery County who say it would run uncomfortably close to some of their homes - perhaps taking a few of the houses - and perhaps threaten their families' health.

The northernmost route, through less developed Howard County, has drawn the wrath of people there, who say it would destroy their area's rural character and lower their property values.

An intermediate route, which would cut less directly through both counties but at several points crosses the Pataxent River, which divides the counties, is being fought by the State of Maryland's Department of Natural Resources. The department said it would destroy the river's scenic beauty and harm an area designed for future park use.

Pepco itself favors the intermediate route across the river. It first proposed this route more than a year ago, then backed off after the Department of Natural Resources, voiced strong objections. But the company's subsequent choice, through Howard County, raised equally strong objections from people there, so the company recently switched back to the river route as its first choice.

"We now feel we ought to go back and fight the state," said Leon Leslie Jr., supervisor of property management for Pepco's real estate department. "It's people concerned about their homes versus proposed parkland. And we try to respond to people."

Perhaps nowhere is the proposed transmission line causing more fear than in the Hanlons' community of Brinkwood. It is there, and in the Brodgen Road area further south in Montgomery County, that the line threatens to come closest to existing homes.

It's not that the people of Brinkwood have never been exposed to power lines. The community already has part of Pepco's 230,000-volt line running smack through it, and, in fact, was planned and built around the line to take advantage of the open space provided by its grassy right-of-way.

The community's two dozen homes, most in the $100,000-to-$150,000 range, cluster around both sides of the right-of-way, and children use it as their main recreation area. The natural greenery of trees and shurbs hides the lines from the view of most of Brinkwoods homes and gives residents a degree of privacy.

But the proposed larger line, Brinkwood residents feel, may destroy the peace they have made with technology.

Dick Dimon, president of the Brinkwood Community Association, is an architect who designed his own home. It is now about 130 feet from the Pepco right-of-way.If Pepco has to widen the right-of-way to install another tower, Dimon may lose some of his front yard. But more than that, he worries that another, larger tower and a widened right-of-way stripping Brinkwood of its greenery might have a "disastrous" impact on the entire community.

"Here's a prime example of a successful residential community that's been developed along a right-of-way for high-power lines," he said. "It's not the type of situation where a community will back up to aline. But the saving grace is that the area is wooded. And that wooded character has to be maintained.

"For a community to be successfully developed around apower line, the lines have to go first. After a community is developed, you have no flexibility. There is no way you can keep it from impacting on the houses."

For some Brinkwood residents, the visual impact causes them less concern than possible health hazards. They have heard about reports from the Soviet Union that electromagnetic fields created by high-voltage lines have caused medical problems for workers around those lines.

And just a few month ago, after three years of hearings on the health and safety of 765,000-volt lines being built in upstate New York, the staff of the New York State Public Service Commission issued a report concluding that "biological effects will probably be induced in humans exposed to overhead lines and . . . such effects may be harmful." The conclusion was based in part on studies showing that exposure to electromagnetic fields caused biological disorders in laboratory animals.

The electric power industry disputes that conclusion. It contends that preliminary results of studies it is sponsoring suggest that the lines hold no hazards for animals or humans. It notes also that 765,000-volt lines have been in use since 1969, and no harmful health effects have yet been observed.

That's not good enough for John Hanlon. "My kids sled ride in that right-of-way, they play ball in it, they run in it, and we're worried about their exposure to the (higher-voltage) line if it goes through," he said. "Things like cancer don't show up for 20 years. Those people who say there's no hazard, they don't know. That's their opinion, and their families won't be living right under it."

Montgomery and Howard County residents facing the prospect of the new power lines note that researchers have studied other problems: the sizzling or humming noise that high-voltage lines produce during wet weather, their possible interference with radio and television reception and even with the operation of heart pacemakers, and the possibility that people who touch ungrounded metal objects under the line may experience painful or hazardous shocks. But, as with the issue of long-term biological effects, there is disagreement over how significant and how controllable such effects are.

In parts of Maryland where the larger, 500,000-volt lines are already in operation, however, many people remain oblivious to them. Some people say they like them.

Chester and Marilyn Gierula, for example, who live next to their right-of-way of a 500,000-volt line that runs a little north of the Brighton-High Ridge section, chose their house partly because of the degree of privacy the power line affords.

"No one can ever build there," said Mrs. Gierula. "And this is an area that is soon going to be overrun with houses." She also likes the fact that she can ride her two horses on the open, grassy right-of-way.

As for side effects, she hasn't noticed any radio or television interference. She can hear the line crackling only if she's outside when it rains. And she considers biological effects "a very remote possibility."

But George and Theresa Grieve, who live near the end of the same line in Frederick County, feel differently. The Grieves can see the line and its huge tower from their living-room window.

They're worried, they say, about the health hazards the lines may pose for their two children. They also resent the line's intrusion on their view and its possible effect on their property's value.

"We eventually want a larger house," said Grieve. "I wouldn't consider building here with that line there, and it concerns me that it might affect the sale.

"I talked to a Pepco engineer who asked me if I appreciated how good the towers look. I said I'd appreciate them a lot more if they had bark and a few leaves on them."

There is not much that people like the Grieves can do about high-voltage line already in their midst. But the people farther south, in Montgomery and Howard counties, say they will fight the construction of lines as long as necessary. They have pooled resources, hired lawyers to represent them and continue to attend seemingly endless hearings on the question.

Sometimes, though, a note of discouragement creeps in. "I've always felt, if it's decided on its merits, the line won't come through this way," said Hanton. "But the longer it goes on, well, I just don't know."