The worst part about the Audubon Naturalist Society's annual family cross-country ski weekend is the letter you get one week before departure.

You get it well after your check (about $200 for a family of four) has been deposited, and you have devoted at least 8 hours to locating, trying on, and reserving cross-country skis from area outfitters.

And well after you have called a family council and decided to believe that this self-confident group of naturalists can -- safely -- introduce you and your two kids (11 and 8) to a sport none of you has ever tried before.

The letter -- strictly business -- tells you that, short of a blizzard, the trip will be held. That "living will be communal, with everyone pitching in to make meals, clean up and chop wood."

And that "privacy will be at a mimimum, with one bathroom and four bedrooms for 20 people (mattresses available for the floor). But we should all be so tired that sleeping will be no trouble."

And the final blow:

"Pack your gear in something that is easy to carry and pack lightly, as we may have to ski in over a mile to the Brewers' house."

"But we can't ski; that's why we are going," I shriek into the phone.

"And we have two young children, one just recovering from the flu."

"Don't you worry. We do this all the time," consoles the instructor in a manner recalling faceless white coats in operating rooms.

"And for goodness sakes," he adds, "don't worry about holding the others back. We're all in this thing together."

The Audubon Naturalist Society does not simply watch birds and brake for animals. it is kind, also, to people.

Thus reassured last year, we closed our eyes and committed ourselves.

One week later, we stood with 12 strangers on a slope in Herrington Manor State Park, in Garrett County, Md. We ranged in age from our 8-year-old to two grandmothers in their late 50s, in attire from fancy to rented.

All of us gazed in awe at Cris Fleming, the Audubon naturalist-teacher who dreamed up these family trips three years ago, as she commanded -- from the bottom of the hill -- "Let 'er rip. Get down here for your lesson!" f

We learned (from blond-bear-of-a-naturalist Allan Morich) first to jump awkwardly from ski to ski, then to slide forward on alternate feet (with a kind of samba kick-glide) without poles, then adding arms and poles.

An hour later (literally), we could ski. Or skid. And soon we took off for a 1 1/2-mile trek through the beautifully groomed trails.

After a time, the rhythm took over and we would slide, bend, slide, without even thinking, at one with the snow and the shadows. High from the exertion, the hypnotic rhythm and the surroundings.

Other images occasionally intruded: My son Mike (the 11-year-old with the flu) flying by with a new group of teen-age friends . . . daughter Evelyn, reaching the limits of her 8 yers, invited by a lovely (and very athletic) grandmother to rest.

The family, on an Audubon trip, is able to soften its hard lines to private identity and blend in with others. A relaxed foolhardiness replaces the usual cautious approach. And everybody gets a chance to play both parent and child. Also, to play scared.

Once, as I waited in an isolated spot with night closing in, I almost abandoned a new friend. A low growl coming from the nearby woods could only belong to a wolf.

It turned out -- I must report -- to be a distant snowmobile.

But the high point of the trip was the vision as we arrived on the hill overlooking our destination, the lights of Audubon members Charlotte and Mike Brewer's farmhouse.

Each of us -- alone -- swooshed down to the dark-on-dark sculptures of barn against blackening snow and sky. Inside: mutual congratulations, a large kettle of mulled wine, and the Brewers, welcoming guests to their simple, but sufficient 150-year-old farmhouse kitchen.

There was a truly insane dinner cooked by half of our group, during which an oversized pot of spaghetti was drained (by my husband, Richard) into the sink, spaghetti (for 20) and all.

After dinner, a hearty group followed leader Cris outside to measure temperature, wind velocity and wind chill factor, and the children wound up tobogganing until 10.

The rest of us settled into the living room to exchange confidences, or to join the Brewers in cracking the black walnuts that grow wild on their 240-acre farm.

Mike Brewer interspersed his cracking with low-key comments about such things as human ecology in a back-country area and family attachment to the "home place" -- a value we were rapidly learning to appreciate.

Soon after 10, the stack of mattresses was dismantled and scattered about. And, sure enough, most of us were so tired that sleeping was no trouble.

The Audubon Naturalist Society's family winter ski adventure (Jan. 24-25), limited to 18 people, is one of a wide range of family outdoor adventures offered by the society, with assistance from Sierra Club representatives. Among other winter activities: Walks along the C & O Canal, orienteering, family nights at the movies.

"All programs," says education director Neil Fitzpatrick, "offer parents and children the chance to learn something together -- as beginners -- for the first time. Hopefully they will continue the activity together as a family after our introduction."