Charles L. Dunn, a man who remembers that in 1940 the annual real estate tax on his 13 acres of Northern Virginia farm land was $19, was more than a little shocked when he received his 1986 tax bill last month.

Fairfax County officials said the value of Dunn's undeveloped land, nestled against the Capital Beltway blocks from the new Dunn Loring Metro station, had increased by about 350 percent in one year.

Dunn, who paid about $9,000 in county real estate taxes last year, calculated that the new assessment meant he owed about $39,000 this year -- more than $100 a day. And there was no mistake.

There was, however, a corollary. Dunn's residentially zoned land, some of which he bought for $100 an acre in the 1930s, is worth more than $2.8 million, the county said.

"Yeah, I'm sitting on a gold mine, but that ain't the point of it," said Dunn, an 80-year-old retired farmer who delights in cursing the county and cackling at his own banter. "You gotta have a damn place to live."

Dunn said he is not interested in selling the land, where he has lived and farmed since the Depression, no matter what the price.

His tax increase is extreme, but not isolated. Property owners in hot pockets of land throughout booming Fairfax County are receiving bills that far outstrip the average countywide increase of 4 percent in residential assessments.

While skyrocketing land values are nothing new in Fairfax, the fallout in higher taxes for homeowners who find themselves in the midst of fast-growing commercial enclaves seems to be intensifying as the county sets the pace for development in the region.

County officials say their assessments do not create value, but simply follow trends in the market and reflect prices in a given area. They acknowledge that certain homeowners are being squeezed.

"The press is now on for residential land that lies in strategic areas," said Harry Middleton, a supervising appraiser for the county real estate assessment office. "If the [owner] was an investor, he should be tickled. If not, people say this is their home and they don't want to sell it at any price."

"Obviously their assets are increasing at a rapid rate," said Fairfax County Executive J. Hamilton Lambert. "I think it's a natural phenomenon if a community has a booming economy. It's the same in Montgomery or Arlington [counties] or Alexandria."

Although property owners can -- and do -- reap huge profits from selling their land in the torrid market, often they are elderly or sick, living on land that has been handed down in their families and interested only in staying put. Many, like Dunn, had no plans for speculating on the land when they bought it, and have no enthusiasm for selling it now.

Mary Lou Curtis, who grew up on a 58-acre farm just east of Fairfax City off Lee Highway (Rte. 29), still pays her share of the taxes her family owes on the land. The farm is less than a mile from the Vienna Metro station, scheduled to open June 7 as the last stop on the Orange Line.

The farm, which was assessed at about $753,000 last year, jumped in value this year by a third, to more than $1 million. Last year, Curtis paid more than $2,000 in real estate taxes for the farm -- one-sixth of the total -- even though she and her husband, a retired Fairfax County firefighter, do not live there.

Some members of her family, unable to meet their share of the tax payments, persuaded her to agree to sell the farm, and it is now under contract. She would not disclose the tentative selling price. Soaring taxes, she said, have forced her family to sever their ties with land they have owned for more than a half-century.

"I love that farm, and it'd break my heart when it starts with houses going up and being developed," she said. "But you know what the assessors tell me: 'If you can't pay your taxes, sell it.' So what can you do?

"I would have loved to keep that [farm] for my children and their children just to enjoy, but it wasn't possible. There's just constant pressure to sell and get out. Everything's closing down around us."

Stories like Curtis' are the flip side of the common assumption that anyone who bought property in what has become a prime commercial area was extraordinarily lucky, or wise, or both.

The same sequence of events that swept up residential land inside the Beltway and near Tysons Corner in the postwar years is now being repeated in Merrifield, Chantilly, around the intersection of Rte. 50 and I-66 west of Fairfax City and near Metro stations, among other locations.

For many, the rising real estate tax bills are accompanied by an onslaught of persistent real estate agents and developers who comb through county land records for the rare remaining parcels of land that can be zoned and developed for large profits. Landowners sitting on prime pieces of real estate say they receive several letters or telephone calls a week from prospective buyers.

"I don't care about being no millionaire," said Dunn, "so I just ignore 'em."

Fairfax sends out about 200,000 reassessment notices every spring. Generally, another 50,000 properties receive no notices because their assessments remain unchanged.

There are a number of ways for property owners who think their reassessments are too high to appeal, and Dunn is planning to take such action. Generally, however, those who appeal have to prove that the reassessments on their land are out of line with the market for property in the area.

Dunn acknowledged that will be difficult, and that he will probably have no choice but to pay the $39,000 the county says he owes in 1986 taxes. Still, he insists, the one-year increase doesn't make sense.

"Of course I can pay it," he said, his pride apparently wounded by the suggestion that he could not. "But who the hell wants to?" CAPTION: Picture, Charles L. Dunn holds 1940 tax bill for $19. The Washington Post