Trout fishermen and would-be trout fishermen imagine, if you can, a limestone trout stream 75 yards long and 25 across wherein rest 8,000 hungry rainbows that average a plump 12 inches and better than a pound.

Imagine that to fish this stream you need to license, that there is no limit on your catch and that you may approach the fish with any form of bait, lure or fly.

Imagine that the only rule you must follow is to take home everything you catch.

And, you will ask, what exactly is the catch?

The catch is that you pay for what you catch, according to a complicated system of lengths and measures under which bigger fish cost more per inch than smaller ones, but the smallest and the biggest of all cost nothing.

This is Fountainrock in Walkersville, Md., just a few miles from Frederick and an hour's drive from Washington. It's the area's only pay-for-play trout stream.

Fountainrock is there because of a 3-million-gallon-a-day limestone spring that bubbles up from the rolling farmland after a 100-mile underground voyage from Carlisle, Pa.

The spring feeds a trout farm where sporting anglers can load up their coolers with fresh rainbow. It's a place for reviving wounded egos after long bouts with warier native tourt, for testing new flies and new presentations, for introducing the wife or the kids to the sport that makes dad a stranger.

The manager of Fountainrock is Marion Burkett, who runs the place the way he sees it - as a farm.

Burkett was a chicken farmer until he discovered one day that you can raise a one-pound trout on a pound of feed. It takes 3 1/2 pounds of chicken feed to raise a pound of chicken, and trout feed is cheaper. Farmers being no fools, Burkett leaped in.

The stream he picked is ideal trout water. The limstone in the water keeps the water at a cool 54 degrees in the summer, and he feeds the fish, which he raises from fingerlings, just enough to keep them healthy and hungry.

All of which makes it just a little too easy for Larry Miller, a stalwart member of Trout Unlimited who has been hooked on the sport from the time he was a tad in the trout paradise of central Pennsylvania.

But not too easy for Miller's wife, Lynn, who got her first taste of her husband's abiding fever at Fountainrock last weekend, in their fourth year of marriage.

"Well, of course she's been busy at work and with Matthew (their 3-year-old)," Miller offered lamely after a thoroughly satisfying and successful day.

"I'm just glad you came," Lynn told the extra party in the outing. "Now he can't yell at me."

As it turned out, Lynn Miller did just fine, working the fly rod her husband rigged for her with a minimum of trouble and a miximum of results. She landed the lion's share of rainbow in a three-hour outing that ended with 13 trout weighing 20 pounds. The bill came to $21.50.

Miller set his charges to work with sinking stone fly nymphs, tiny flies that descend slowly from the surface and are retrieved with gentle jerks until a trout takes hold. Plenty took hold.

Miller has caught enough trout that it was no challenge, so he set his sights on a grey torpedo that lurked in the deepest hole. It was a six-pound, 22-inch rainbow that Burkett said would be free if anyone could land it. Also free are any little ones under eight inches.

Miller hooked that monstrous beast after and hour of shaking off littler fish that beat the lunker to the fly.The thrill was brief.

"I just got too excited," Miller said after the prize had shaken itself off the hook with a powerful sweep of its mane.

Not too many get off Miller's hook. Good thing his wife was there to see it the one that got away.

The National Capital Chapter of Trout Unlimited will hold its regular monthly meeting at 8 o'clock tonight in the National Wildlife Federation Building, 1412 16th St. NW. The speaker will be Ben Schley, who will talk on Potomac River smallmouth bass fishing.