For more than four months, three Interior Department officials have pored over scores of desk-size maps, drawing lines that could make the difference between million-dollar profits and million-dollar losses for property owners along hundreds of miles of Atlantic and Gulf Coast beaches.

The work was required by an obscure section of the budget reconciliation law barring federal flood insurance policies on "undeveloped coastal barriers" after Oct. 1, 1983. The law gave Interior the job of applying the definition to 3,800 miles of coastline from Maine to Texas.

Everyone involved agrees that when the maps, released last week in preliminary form, become final, they will have a major impact.

For a Republican administration that tends to favor development, a leaner budget and a smaller role for the federal government, the mapping process presented a dilemma. Eliminating flood insurance on the sandy spits of land would cut back flood insurance payouts.

But it could also limit growth and alienate developers, many of them staunch Republicans. Without federal flood insurance, few bankers will provide mortgage money in areas that lie in the path of fierce ocean storms.

With millions at stake, lobbyists spent the fall shuttling to and from Interior's headquarters. The greatest pressure came from the Southeast, where more of the coast is still open to development. Locally, the impact was more limited: the biggest fight involves 200 acres in North Bethany Beach, Del.

Among those who took an interest in the process were former Massachusetts governor Endicott Peabody, whose client, Sea and Pines Inc., owns more than 75 acres in North Bethany Beach. Energy Secretary James B. Edwards, a former South Carolina governor who owns a home in Mt. Pleasant, a Charleston suburb close to the coast, and has many friends among coastal landowners, also kept a close watch on the mapping, according to Stephen Jones, his administrative assistant.

Edwards hosted a $1,000-a-couple fund-raiser for the South Carolina Republican Party Nov. 6 with his weekend house guest, Interior Secretary James G. Watt, as the drawing card.

At Edwards' urging, the pair flew along the coast from Rehoboth Beach to Charleston, looking at the barrier formations along the way.

These people and a dozen or more landowners and lobbyists brought to Interior geologists' reports, aerial photographs and maps to prove their property did not qualify as an undeveloped barrier. At the same time, environmentalists argued that most of the disputed property did qualify.

The lobbying was unpredictable; little was heard from Florida although 143 miles is affected. Some Texans worked hard to avoid inclusion of areas like Matagorda Island, with little success.

After several weeks work, the members of the Coastal Barrier Task Force settled on definitions. An area was developed if it met one of three criteria: it had more than one structure per five acres, it had a complete system of roads and sewer and electric lines, or it belonged to a developer who had put utilities and buildings on part of the property and had plans to develop the remainder.

The third criterion--"phased development" -- infuriated environmentalists. Calling the proposed definition "outrageous," Sharon Newsome of the National Wildlife Federation said, "Interior has ignored specific directions from Congress and has issued a proposal that would designate as 'developed' pristine coastal barriers . . . "

When the definitions were applied to maps and released a week ago, landowners and developers found out who had won the first round of the rulemaking fight.

In Delaware, Peabody's client and another landowner, Carl Freeman and Associates of Bethesda, lost. Their land was designated an "undeveloped barrier," though Interior noted there were conflicting expert opinions. Freeman's firm had plans for a high-rise development; Sea and Pines had no immediate plans but wanted to preserve its options, according to Peabody.

"There's no way that area could be considered a coastal barrier," Peabody said. "If they want to preserve the land, they should just take it."

(Government condemnation would mean compensation for property owners; an undeveloped designation provides no compensation.)

In Maryland, no land is affected. All 31 miles of the coast are protected or developed. In Virginia, 16 miles of beachfront were designated undeveloped in the proposed maps, including 931 acres on Assawoman Island, 1,216 acres on Metomkin Island and nearly 9,000 acres on Cedar Island.

In Mississippi, three islands--Round, Deer and Cat--received an undeveloped designation. Nathan Bodie, whose family owns nearly 2,700 acres on Cat Island, says that the designation will cost him money but will not derail his plans. If the designation becomes final, he says, he will build on higher ground and gear his marketing to affluent customers who could afford an uninsured loss.

In Louisiana, state officials took the lead in fighting to keep their coastline off the maps. Louisiana gets substantial income from offshore oil and gas drilling, as long as the oil and gas is found within the three-mile limit. But the shoreline is eroding. Without federal funds to pay for stabilizing engineering work, revenues might literally drift out to sea.

A pending bill sponsored by Sen. John H. Chafee (R-R.I.) and Rep. Thomas B. Evans Jr. (R-Del.) would prohibit federal expenditures for roads, sewers and bridges in undeveloped barriers.

Louisiana officials worry that the flood insurance maps would be grafted onto that legislation.

In South Carolina, consultant Jim Scott had reason to be satisfied with the maps. Among Scott's clients and the dozen members of his Coastal Properties Institute, formed last summer with charter members paying $10,000 each, are the owners of Kiawah Island Co. Ltd. This firm, wholly owned by the Kuwait Investment Co., has invested $60 million in the 10,000 acres of Kiawah. Most of the island was designated part of a "phased development."

"People keep focusing on the developers. There are thousands of property owners who own a lot on a piece of property being circled on the maps ," Scott said. "Some of these are Mom-and-Pop operations . . . and they don't know that they won't be able to develop," he said. "Everybody thinks this is affecting fat-cat developers and nobody else."