Harry Leslie of The U.S. Forest Service remembers when he used to have to send out press releases every fall reminding people free firewood was available at national forests. Needless to say, he's saving a lot of money on press releases these days.

It's estimated that half a million families around the country took advantage of the Forest Service's offer last year, hauling away $2.6 million worth of free firewood. And the demand for the wood is growing "in quantum leaps," Leslie says.

Everybody wins. Consumers can indulge in Bunyanesque fantasies while saving up to $100 a cord on wood, and the Forest Service doesn't have to spend money harvesting dead trees that could hinder new forest growth or cause fires.

To heat an average house in this area for the winter takes from three to five cords of wood. A standard cord (a stack of wood eight feet long by four feet wide and four feet high) can cost from $50 to $100 delivered and stacked.Hence, the popularity of cut-your-own.

But you've got to think it through. "When you add up the cost of getting to the forest, and remember you're driving a pick-up truck that gets rotten mileage, 'specially when it's loaded up, and then pay for a chain-saw to cut the stuff down, the whole thing can quickly become uneconomical," Leslie says.

He says many people just don't think. "They don't know what a mess it can be. When I was a kid in Wisconsin we heated with wood. It was the kids' job to haul the stuff into the house and stack it. I hated that. And the ashes -- what're you gonna do with all those ashes? The garbagemen'll love you when the bag splits open. You can't put ashes on your lawn, they'll ruin it. You could put them on your sidewalks, but then you track 'em into the house. And then you've got to be careful with the storage: You could bring termites or carpenter ants into the house.

"If you know what you're doing it's great, but you can make enough mistakes to make you sorry you ever tried it." WHERE THE WOOD IS First of all, forget about Rock Creek Park. "We start getting requests about this time every year," sighed a National Park Service spokesman. "There is no firewood available in the national parks, period. If a tree falls in the stillness of the woods, we just leave it there to recycle."

Instead, try construction sites, dumps, saw mills, highway and power-line maintenance crews, county extension agents, private land and park lands.They can be good sources for free or inexpensive wood once permission has been obtained. Local newspaper classified sections carry wood listings.

In this area the Lee District Ranger for the George Washington National Forest in Edinburg, Va. (703/984-4101), and Maryland State forest officials at Cedarville State Park in Brandywine (301/888-1622), will issue wood cutting permits depending on the amount of "downed or dead wood" available in their areas. The cost for a pickup load (about half a cord) is $6 in GW Forest and $5.25 at Cedarville, and it's B.Y.O. pick-up truck and chain saw.

At Maryland-National Capital Park and Planning Commission in Upper Marlboro they'll cut and stack the wood for you, and you pick it up and load it. Cost: $50 a cord. Call 301/952-1477 to get on the waiting list. Plan on a three-to four-week wait.

Other state forests in Maryland and Virginia have cut-your-own programs, but the distances involved make the wood a questionable value, unless you're coming back from a trip or something. For a list, call the Virginia State Forestry Office in Richmond, 804/977-6555, or the Maryland State Forest Service in Annapolis, 301/269-3776.