Since the end of World War II, rural residents of the easternmost end of Long Island have been warily eyeing the inexorable creep of suburbia from the outer boroughs of New York City through Nassau County toward picturesque Montauk Point.

Fearing the flood of housing developments, shopping centers, fast-food stores and parking lots that has swept over the western half of Long Island since the 1950s, farmers, summer homeowners, environmentalists and politicians in 927-square-mile Suffolk County began to show their concern in bizzare ways.

They proposed the creation of a new county dedicated to fighting development, which variously was given the working name of Atlantis - for the fabled underwater city - or Peconic, for the region's vast salt water bay.

Once it was even seriously suggested that eastern Long Island secede from New York State or even the United States in order to repel the onslaught of humanity, which since 1960 has propelled the county's population form 600,000 to 1.3 million. Planners predict 3 million by 2020.

Now, as if throwing up the last barricades against sprawl, Suffolk County has decided to invest heavily in agrarianism.

Suffolk County Executive John V. N. Klein will begin negotiating this week with hundreds of farmers for the purchase, in effect, of the right to preserve 3,883 acres as farmland.

The cost will be $21 million to start, and if the "rogram is succesful, the county will buy agrarian commitments from hundreds more farmers at a total of $54 million.

The technique Suffolk County is using - along with New Jersey and several others states surrounded by urban sprawl - is the purchase from farmers of development rights that usually are sold to builderes, sometimes along with land titles, and sometimes with a rent-back option that lasts until housing needs warrant building.

Suffolk County has decided to buy, for an average $5,000 an acre, first options for development of farmland, trading the ivestment for permanent guarantees that the land will remain in agricultural use. The county is issuing 30-year bonds to raise the money.

New Jersey has received offers from about 100 farmers for development rights to about 15,000 acres. Moreover a congressional subcommittee recently held hearings on a proposed agricultural land policy act that would appropriate $200 million for experimental projects in preserving farmlands.

Under the development rights techinque, the government not only preserves the esthetic quality of rural areas, but sharply reduces the public services it would have to provide if the land were sliced into housing tracts.

Although suburbanites for years have subscribed to the axiom that more tax-paying households will lower per capita levies, community planners now acknowledge that a rapid rise in land development increases in local taxes because of costly new roads, sewer systems, schools and other services.

Summer homeowners have opposed new development because of the esthetic effect of subdivisions on the fields, which are kin to the New England countryside, visible across Long Island Sound.

But the county's farmers, who have seen 100,000 acres of productive land reduced to barely 50,000 in the last 20 years - and some of that 50,000 on short-term lease, have cited constantly narrowing profits margins when they plead for tax relief.

So there may be added incentives to farmers who join the development rights program advanced by County Executive Klein, a Republican who rammed his idea through the Democrat-controlled Legislature last September.

The value of land that can be used only for farming cannot be driven up by speculators. That protects it not only from skyrocketing property taxes but from confiscatory inheritance taxes. Thus the farms stand a better chance of being passed along from one generation to the next.

Besides, the housing market in Long Island has been generally depressed in recent years, while price of its farm produce has slowly increased.

"Even though Suffolk has lost a good deal of productive land in recent years, it still is the leading agriculture producing county in New York State," noted Lee Koppelman, director of the Nassau-Suffolk Regional Planning Board.

The county crops, mainly potatoes and cauliflower, have a product value of nearly $100 million annually, he said.

Early in the 1950s, Koppelman recalled, Suffolk farmers were dumping unsold potatoes in Long Island Sound because of depressed prices; now, potatoes are selling for $7 a hundred weight, and it is the housing industry that is stagnated, Koppelman said:

"The question to ask is, if agriculture goes out of Long Island, what is supposed of supplant it? Is it going to be something that is a drain on the tax base, or something that adds to it? In effect, the public is saying it is cheaper to buy the development rights to farms than to spend unknown amounts of money for urbanization."

Koppelman said a "floating constituency" of Klein supporters, led for the most part by environmentalist, summer homeowners and tourism developers, have actively argued that urbanization of eastern Suffolk would hurt tourism as well as the quality of life for year-round residents.

Farmers of the easternmost north and south forks of the county - which split off from the county seat of Riverhead - even have begun buying back land from housing speculators because of "a feeling that things may have been turned around," Koppelman said.

Large amounts of Suffolk's remaining farmland, however, are already in the hands of speculators, most of whom lease it back to the farmers for about $100 an acre while awaiting the right market conditions for development.

Yet, "people who came to Suffolk County view the agricultural land as a recreational resource," Klein argued. "They like to look at it. Wall-to-wall houses aren't an esthetic attraction.