For Jack and Gladys Chocola, the 16 acres of woods they own in Fairfax County's prestigious Great Falls have been their inflation-proof nest egg. Mrs. Chocola calls the acreage "our informal pension plan," her husband calls it "our pot of gold at the end of the rainbow."

Over the coming year, shortly before Chocola's retirement as a pharmacist, Chocola and his wife, like other Great Falls landowners, had planned to divide their prized parcel into two-acre lots -- perhaps as many as seven. With the price for two-acre lots running from $35,000 to $50,000, the Chocolas could reap $250,000 or more on their 14-year-old investment of $27,500.

But now, they fear that their pension plan is in serious trouble, that the bottom may have fallen out of the pot of gold.

Under new zoning being considered by the Fairfax Board of Supervisors, the Chocolas, along with about 1,100 other landowners in Great Falls and near the Occoquan Reservoir at the other end of the county, would be barred from dividing their property into lots smaller than 10 acres.

More than 4,700 other landowners in the same two areas would not be able to divide their property into lots smaller than five acres.

County planners say the massive rezoning to lower densities -- affecting almost 55,000 acres, or a fifth of the county -- is necessary to save Fairfax County from more urban sprawl and preserve the environmentally sensitive Great Falls and Occoquan areas.

The proposed changes on the two relatively undeveloped areas could have a major impact on the future growth of the Washington suburbs. Fairfax County currently accounts for almost half of the region's new housing starts and government studies indicate that trend should continue, with much of the projected growth in the Great Falls and Occoquan areas.

But many of the landowners who would be affected say the rezoning would exact a high price -- and they would have to pay it.

"If I can't subdivide," says Jack Chocola, "it will be a financial disaster for me."

Scores of landowners, including the Chocolas, took their grievances to hearings held last week before the Fairfax Planning Commission, which yesterday recommended that the supervisors withdraw the so-called down-zoning plan. Many of the commissioners support some sort of down-zoning, but feel that the present plan in inconsistent and too drastic.

The supervisors, who can accept, reject or modify the recommendation, will hold hearings of their own this week, and are scheduled to make their decision on Dec. 18.

A frequent warning at last week's hearings concerned possible legal action, considered inevitable if a down-zoning plan goes through. There is a precedent -- Fairfax Board of Supervisors vs. Carper -- a 1959 decision that is to Virginia land-use cases as Brown vs. Board of Education is to school desegregation.

A Great Falls landowner, G. Wallace Carper, successfully fought that ealier attempt by the county to down-zone the western two-thirds of Fairfax.

County strategists think down-zoning will work this time because it is not as sweeping and because, among other reasons, it is in harmony with the comprehensive plan adopted by the county in 1975.

But William H. Hansbarger, a chief strategist in the 1959 case and an attorney whose white mane and shrewd legal mind have earned him the sobriquet "the silver fox" among some county staff members, listened to a county planner last week trying to justify down-zoning legally, then said with resignation, "I think I'll have to study up on Carper."

Again and again at the hearings, landowners -- a few of them large-scale developers or speculators, but most of them holders of small parcels of 10 to 25 acres -- said down-zoning was unfair. A frequent refrain went this way: "I've had two-acre zoning for years, and paid taxes on that zoning, and now you're saying I can't subdivide into two-acre lots, only five- (or 10-) acre ones."

However, a survey of the land records of many of the complaining owners showed they have, over a period of years, been paying relatively low taxes based on what even the county assessor's office calls "conservative" assessments.

For example, the Chocolas, who say they could subdivide their peoperty into six or seven two-acre lots that could yield a total of $250,000 or more, are assessed at only $80,985.

As desirable land in Fairfax irretrievably shrinks, the incentive to break the present pattern of five-acre development in Great Falls and Occoquan and subdivide into two-acre parcels has become stronger.

The supporters of down-zoning are too polite to say so, but some of them acknowledge privately they think that land investors have been spoiled by the increasingly bullish market in the Great Falls and Occoquan areas, where the risks have been low and the potential for profit high.

Supervisor James M. Scott (D-Providence), the earliest advocate of down-zoning among board members, said: "No matter what the final development is, the landowner is going to make a substantial profit. Is the county obliged to guarantee themm a maximum profit?"

But legal criticisms of down-zoning point out tht the county took 3 1/2 years to get around to down-zoning since adoption of the comprehensive plan.

Supervisor Scott wanted the board to grapple with the issue 2 1/2 years ago but he found no support. Some down-zoning critics find it curious that the issue has come to the surface shortly before the election campaign for the nine board seats gets under way.

"They (the supervisors) are trying to put a political trophy on the wall," one zoning attorney, who did not want to be identified, said.

But while down-zoning, if enacted, would win applause, it would also -- to judge from last week's three hearings -- evoke intense opposition.

Some of the most intense opposition would come from Great Falls, even though the 300-family-member Great Falls Citizens Association, by a 40-to-17 vote, endorsed down-zoning a week and a half ago. Even though many more of his constituents have opposed it at the hearings and in signed petitions, John P. Shacochis, Republican supervisor, remains a firm supporter. Besieged at a recent meeting by angry constituents, he said:

"Down-zoning has a nasty taste to it... But it is a process we're going to have to go to if we're going to keep growth in this county balanced."

Meanwhile, zoning attorney Hansbarger and his colleagues are reading up on "Carter."