The day was perfect - a slight breeze, bright sun, autumn leaves still on the trees, tall grass yellowing -and on the way back to Shanghai I stretched out in an elm grove. I could look toward the road and see farmers and wagons, silhouetted at the edge of their fields. Or I could look at the sky and know I was alone, removed not only from the West but from every Westerner in China, and feel utterly secure in the solitude. That this day's trip turned out to be my last extensive one stamps the image more clearly in my mind.

The next morning our group flew to Canton, where I secured a bicycle only for short loops through the city. Four days later, we boarded the train for Hong Kong, There, leafing through the first English language newspaper I had seen in weeks, my eyes alighted on a tiny Reuters item. It was the latest official count of bicycles in China - 77 million. Minus one rider, I thought to myself. If You Go ... ... to China with the intention of touring by bicycle, you can take your own bike, buy a Chinese model or travel with a bikers' tour group. Bike rentals are not available in China.

Travelers who want to take their own bikes must pay air freight. Pan American World Airways, for instance, charges $3.78 a pound for cargo on its New York-Peking route. A 10-speed bike can weigh as much as 35 pounds, and the per-pound cargo rate may rise if the bike is packed in a large container, using up more of the plane's luggage space.

A Pan Am cargo agent suggests breaking down the bike as much as possible - at least removing or turning the handlebars sideways, and removing the front wheel. Loose parts should be lashed to the frame and the entire machine well padded and then packed in cardboard. Pan Am does not provide packing materials for bikes.

But the low price and high quality of Chinese bicycles make it advisable to purchase one in China, use it there and then bring it home. The majority of Chinese bikes do not have gears and look somewhat bulky, but they are as swift and seemingly indestructible as their brand names imply - Forever, Phoenix, Flying Pigeon. I saw models selling for between 149 and 172 yuan (about $104 to $121) at several stores catering to foreigners. The logical starting point for a purchase is the Friendship Store located in most major cities.

American Youth Hostels operates cycling tours of China, with roughly one departure each month throughout 1981. The basic trip lasts 16 days and is centered in Canton; extensions of up to three days and visits to Shanghai or Peking are available. Two Chinese guides travel with each group and nights are spent in hotels. Participants must be 18 or older, unless accompanied by their parents. Prices for the tours range from $2,575 to $2,982, including everything but three meals in Hong Kong. For information, write American Youth Hostels, 132 Spring Street, New York 10012 (212-431-7100).

When you are biking in China, you will find that crowds and confusion pose the greatest risk. It is a good idea to carry a map and, as a precaution, a piece of paper with your hotel's address written in English and Chinese. English language maps are available at most hotels and foreign language bookstores for about 35 cents. But the romanization of street signs is not yet complete, so it helps to steer by landmarks.

China gives riders several legal and tactical advantages over motorists. The far right lane of major roads belongs to bikers. In any collision with bicyclists, motorists face the burden of responsibility and that threat forces them to drive with discretion. Payment to an injured biker is said to be a common settlement for an accident; the only two crashes I saw, both minor, brought policemen to berate the driver involved.

Virtually all urban streets and most major rural routes are paved. Even the oldest of Peking's narrow alleys and back streets seems to sport a coat of asphalt - not much for historical integrity, but a relief to a bicyclist fearing cobblestones, gravel or dirt.

Security is not a major problem. Chinese bikes come with a key lock that brakes the rear wheel. In the major cities one can leave a bicycle with a watchman for one and one-half cents. Maintenance is handled by repairmen who set up shop along boulevards and alleys. - S.F.