A plan by the Potomac Electric Power Co. to build high-voltage transmission cables through parts of Montgomery and Howard counties has sparked fierce community opposition there.

Residents along a 10-mile stretch, starting in Brookville in northern Montgomery and ending in Scaggsville in southern Howard, have asked zoning appeals boards in their counties to reject Pepco's request for variances to construct a $20 million 500,000-volt overhead power line through their communities.

The residents say they are worried that the electrical and magnetic fields generated by the line could cause cancer and other health problems. In addition, the residents fear that the 148-foot-tall steel towers that would be used to carry the cables would decrease the value of their homes and land, some of which backs up to the proposed towers or is in the middle of the proposed route, according to Vince Guida, an attorney representing land owners in Howard County.

The utility has been buying 150-foot-wide tracts of land in Howard since last March for a right-of-way to build the towers, which would cut across 6 1/2 miles of housing subdivisions and rolling farmland to complete one of the last links in a 243-mile regional electric power loop surrounding the Washington area. So far, 200 miles of the loop have been completed.

In Montgomery, the utility intends to run the segment, known as the Brighton-High Ridge line, through an existing 250-foot-wide right-of-way currently used for four 230,000-volt lines.

Nancy Moses, a Pepco spokeswoman, said the loop -- a joint venture among the company, Baltimore Gas & Electric Co. and Virginia Power -- is essential for the utilities to exchange power among themselves and with other utilities in the eastern United States.

It also would enable Pepco to keep pace with the growing demand for power in the area by boosting the utility's transmission capacities. Pepco hopes to begin construction on the line in 1991 and to start operating it in 1993, Moses said.

The latest struggle before the Montgomery and Howard county zoning appeals boards, which recently began holding hearings on the proposed line, caps a 10-year controversy and adds to a growing concern over the safety of high-voltage power transmission lines throughout the country.

Pepco first applied for permission from the Maryland Public Service Commission to build a 500,000-volt line to complete the loop in 1977. After spending three years examining seven possible routes for the line, the commission approved a plan to build an overhead line from Pepco's Brighton substation in Montgomery to the utility's High Ridge substation in Howard.

That decision prompted Howard County and homeowners there to challenge the commission's decision in a suit filed in circuit court, which upheld the plan in 1985.

Last March, Pepco applied for special height exemptions for the towers, said Elizabeth Entwisle, Howard County's assistant county solicitor. Montgomery's planning board has recommended that its zoning appeals board approve the line, while Howard's planning staff has suggested denying Pepco's application.

Recent studies have found a possible connection between childhood cancers, increased stress and other health problems and the electromagnetic fields generated by high-power transmission lines. In particular, a report issued last July by a New York State Health Department panel that examined the safety of power lines found that as many as 15 percent of all childhood cancer cases could be attributed to magnetic fields.

Judy Iager, whose family owns a Howard County farm that lies in the proposed line's path and who is paying much of the $5,000 to $10,000 in attorneys' fees for the zoning fight, said she is concerned for the health of children in her family.

"I have three boys and nephews and nieces. I don't worry about it as much for myself as I do for them," Iager said. "I've read studies done over the past 15 years throughout the world showing the dangers of high-voltage lines. And they still don't know the full biological effects. Everything we've read says the subject needs further study."

She said she also worries that the electrical fields generated by the line could disrupt the milk production of her 350 cows. "Cows are highly sensitive to electricity," she contended.

Iager said her family would have to "work around the power lines every day" because the power line would "run through the middle" of the 1,000-acre farm.

Robert Murch, president of the Brinkwood Community Association, a 31-family homeowners' association in Montgomery, said that even though the 230,000-volt lines already run through the community, residents worry that the 500,000-volt line will present more of a danger.

"We're convinced the line is a health hazard," he said.

His group repeatedly has asked the county's Board of Appeals to remove itself from the case, claiming that it lacks the expertise to judge the safety questions. The association recently wrote to Montgomery County Council President Rose Crenca requesting that the council intervene in the matter.

Susan Lawrence, a homeowner in the Murray Hills subdivision in Howard, said she worries about the effect the transmission lines will have on property values. "It's almost better to move out before the towers go up and detract from the appearance of my house and its value," Lawrence said.

Michael Dechant, a Rockville real estate appraiser, said that "there isn't an overwhelming stigma that goes along with houses that sit next to high-powered lines. ... But if you look at a comparable house elsewhere, I would say the house without the power line would be worth more."

Pepco's Moses said the power line would present no health hazard.

"Our experts have been testifying before zoning boards that there is a vast body of information that says there is no major concern," she said. "No direct relationship between ill-health effects and power lines has been found to date. All of our lines are constructed and monitored to meet national electric safety codes."

Edwin Lawless, Pepco's civil engineering manager, testified before the Howard zoning appeals board that the height of the transmission towers and the width of the right-of-way should provide a large enough buffer to eliminate any possible hazard from the electrical and magnetic fields created by the power line.

Moses also disputed any impact upon property values. "There are a number of land owners and property owners who like to live next to right-of-ways because they have open space behind them. There's just no chance of a property value drop," she said.