There's lots of good chemistry going on out there. A rush of botanical juices, from leaf veins back into stems, brings out the pigment that's built up all summer. While recent rains may delay the action, this horticultural blush won't be able to help itself as the day get shorter.

What we're all watching for is a loss of chlorophyll. Then, says Francis Gouin, what's left of the carotene (yellow-orange), xanthophyll (orangeish-red) and anthocyanin (reddish-purple) -- makes for coloration: the chemicals break down and bring out pigments when the leaves fall down. Or sooner.

Ergo: fiery hills of red, yellow and gold; cars iching along Skyline Drive, bumper to bumper at Great Falls; crackling flakes of colar raked into mounds, pounced upon by knee-biters. Dogwood, sumac and white oak run to red; poplar, beech and birch faor yellow. It's a sweet-smelling chemical reaction.

Best to head for the hills: to Catoctin Mountain State Park for maples, to Sugarloaf Mountain for a tri-state foliage vantage (Virginia, Maryland and West Virginia), to the Shenandoah, patiently or on a weekday. Virginia State Parks naturalist Tom Blackstock recommends Douthat State Park near Covington (703/862-0612) or farther from town, Grayson Highlands State Park at Mount Rogers (703/579-7092), for spectacular fall vistas. Lower altitudes offer C&O Canal barge trips and autumnal festivals, like Front Royal's Festival of Leaves in two weeks.

Dryness of summer determines brightness of fall: hope for enough moisture to keep up the good vegetative growth, but not so much as to yield a muddy color. The water the season, the more muted the colar.

Gouin, professor of ornamental horticulture at the University of Maryland, wouldn't want to predict the peak weekend. Who knows? Hurricanes could tear all the leaves off or cells could take their sweet time decomposing. But generally, he says, fall colors reach their primes within three weeks of past peaks. That would mean optimal viewing in early October for Western Maryland, mid- to late October for the central part of the state. The Blue Ridge Mountains should be ablaze around the fourth week of October. Early reports from the north say New England already has hints of color, so chances are it will all come off on schedulte. Stand by for a change of season.

LEAF WATCHING

ON THE RIVER -- River and Trail Outfitters suggest getting off the highways and onto the rivers to catch the spectacle. a half-day raft trip -- no experience necessary -- on the upper stretches o the Shenandoah River through Harper's Ferry costs $28 on weekends. The six-mile introductory whitewater trips for beginners start Friday (25th), continuing through the second week of November. Weekday trips,scheduled when 12 or more people sign up, cost $18. Alternative canoe routes are offered for skilled river runners. An hour from the Beltway. Exit on 340 W in Frederick for 16 miles, and take the last right before Potomac River bridge.

IN WEST VIRGINIA -- The Mountain State Forest Festival, in Elkins, West Virginia, September 30 through October 4, has the distinction of having been attended by Presidents Nixon and Ford. Contact Box 369. Elkins 26241. 304/636-1824.

IN PENNSYLVANIA -- The Fall Foliage Festival in Bedford, October 3 & 4 and 10 & 11, offers foliage tours, a parade of 200 antique autos, a fiddling contest and the Shawnee Rangers. Fall Foliage Festival Committee, Bedford 15522. 814/623-1771.

IN THE CATOCTINS -- "Fall Colors in the Scenic Catoctins," 8:30 to 6:30 on October 25, combines the history of the region with a driving tour through old Fort Ritchie, Blue Ridge Summit, through the Catoctins and South Mountains. It's $32 for Smithsonian Resident Associates, $42 for nonmembers. Call 357-3030.

WITH INDUSTRY -- The C&O Canal history tour of Lanaconin and Mount Savage October 31 emphasizes early American industry, $33 for Associates, $43 for non-mebers.Call 357-3030.

AT FRONT ROYAL -- The Festival of Leaves, October 10 and11, will have pony rides, arts & crafts, antiques, a Civil War battle re-enactment -- not to mention the leaves. It's $3 for adults, $1.50 for students.

IN MARYLAND STATE PARKS -- Fall color walks led by Park Rangers at 1 p.m. in several parks require advance reservations: Oct. 10 and 11 at Deep Creek Lake, Garrett County (387-5563); Oct. 18 at Rocky Gap. Allegheny County (777-2138); Oct. 18 at Fambrill, Fredercik (473-8360); Oct. 25 at Patapsco, McKeldin area. Metropolitan Baltimore (747-6602). Call Barbara Newnam. 301/269-3761 for other park information.

BY BARGE -- The C&O Canal clipper barge departs Great Falls Park, Maryland, at 10:30, 1 and 3 on Friday, Saturday and Sunday. Trips last about an hour and a half. Tickets may be purchased two hours in advance at Great Falls Tavern. Adults $3, under 12 and senior citizens, $1.50. (Private groups can charter trips on Wednesday and Thursday.) Call 299-3613.

ON YOUR OWN -- Frederick County offers a selfguided driving tour book by mail. Send $3.25 to Tourism, 19 East Church Street, Frederick, Maryland 21701.