Most hunting and fishing types agree the toughest challenges in the East are fly fishing for native trout and hunting elusive, wild ruffed grouse.

So it comes as something of a surprise, and no small comfort, that the Interior Department's new assistant secretary for fish, wildlife and parks has two passions -- fly fishing for native trout and hunting grouse -- and pursues both in the most challenging places, public land and public water. It's also refreshing that, like the rest of us, Bill Horn comes away empty sometimes.

"That's it," Horn fumed as he disassembled his rod on Spring Creek near State College, Pa., last weekend. "The absolute nadir. If anybody told me I'd spend three days in June fishing Pennsylvania limestone streams and go home skunked, I'd have said they had rocks in their head."

Horn, whose fortunes were plagued by thunderstorms night after night, was still muttering a few minutes later when his fisheries chief, Joe Kutkuhn, hollered from below, "Hey, Bill, don't quit yet. Something's coming off."

A long-awaited insect hatch was occurring and trout finally were rising to take bugs on the surface. Horn, who had fished nearly nonstop for eight hours, rerigged and hurried to the bank, now shrouded in darkness.

Fifteen minutes later he'd landed a 15-inch wild brown trout, which he released unharmed. The three-day skunk was exorcised and happiness was all around.

Why, you might ask, is a government official one step away from cabinet level, a guy with 20,000 people working for him and access to some of the richest waters in the nation, fishing his brains out at night alongside John Q. Public on a difficult stream four hours' drive from home?

Because that's what Horn likes, which spells relief for many who worried through earlier, less appealing Reagan administration appointees.

"I'm pleased to have him as a successor," said Bob Herbst, who ran fish and wildlife in the Carter administration and was a critic of James Watt, since replaced as Interior secretary by Horn's boss, Donald Hodel.

Herbst said public disaffection with "Watt & Co." may have convinced the administration that environmental policy was a bad place to pursue confrontational politics. As for Horn, said Herbst, "He's too high-principled to see the environment destroyed."

Horn loves his new station ("greatest job in the world," he said), but sometimes finds it hard to get away. Last year, before he was confirmed as assistant secretary in August, Pennsylvania Fish Commissioner Ralph Abele invited him to fish Penns Creek, pristine public water so deep in the woods it remains one of the best trout streams in the East.

Abele assigned state employe Fred Johnson to guide Horn, but when Johnson started out with a tactics lesson, Horn cut him off.

"I said, 'Look, I expect the Grannoms to hatch this evening and I've tied up some harewings. I'm ready to go. You don't have to lead me around. I've been coming up her for 10 years.'"

Said Horn, "I think Fred expected one of these typical senior government guys who fishes twice a year and looks like he just stepped out of the Orvis catalog."

After three days fishing last weekend until bitter dusk, short nights of sleep, camp-cooking for five, tying flies, forgetting to shave and downing a dram or two of Glenfiddich before bed, Horn was a long way from an Orvis catalog.

Horn, 35, said he started fly fishing when he was a teen-ager in New Jersey. "Four of us went," he said. "One mom would get up at 5 a.m. and drive us to the stream, another picked us up that night at 6. In between, you were on your own. I fell in one day, and I can remember standing in the 40-degree rain in my underwear, wringing out my wet clothes. We still had four hours to wait."

Horn went to American University, where in 1972, his senior year, he took up fly tying. That year he helped resurrect the flagging National Capital Chapter of Trout Unlimited, where he remains a member, and started its publication, Riffles, which continues to prosper.

Last year, after more than a decade in environment-related jobs in Interior and on Capitol Hill, he replaced Ray Arnett, a bear of a man who alienated many by taking an intractable stand against steel shot. Horn, by contrast, said he expects within five years all U.S. waterfowl hunters will be using steel shot insted of lead, which scientists say poisons waterfowl.

Horn's major fishing initiative so far has been to direct the department away from raising fish in federal hatcheries to stock streams and private ponds, and concentrate instead on restoring declining coastal and Great Lakes fish, such as striped bass and lake trout.

So far, he's won high grades from environmentalists. Their only concern is that as a relative newcomer, he might lack clout. His two top assistants, Park Service Director Bill Mott and Fish and Wildlife Director Frank Dunkle, are veterans with close links to the White House, and some worry about their loyalty to a young assistant secretary, should a confrontation arise.

It doesn't bother Horn. "I get along fine with my deputies," he said. "I have no inferiority complex."