TAIPEI, TAIWAN -- Residents of Taiwan returning to China to visit for the first time in almost 40 years are being warned to bring plenty of gifts for their less fortunate brethren and to have a dental checkup before they leave "because there are at least one billion cavities on the mainland waiting to be treated."

The advice, dispensed in guidebooks hawked by entrepreneurs here, reflects the interest that has been generated following the Nationalist government's decision to lift a 38-year-old ban on travel to the mainland. Since the ban was lifted Nov. 2, hundreds of thousands of Chinese who fled to the island of Taiwan after the communist victory on the mainland are now flocking to China to visit their old homes and relatives.

Taiwan officials insist that the lifting of the travel ban is strictly for humanitarian purposes, but some observers believe the move will lead to a much broader opening by Taiwan to the mainland.

The Nationalist government was under heavy pressure from its own citizens to lift the ban, particularly from Army veterans, who have formed several activist groups.

But aside from the humanitarian considerations, many observers believe the Taiwan government, led by President Chiang Ching-kuo, has several aims in ending the ban. They say Chiang and other officials want to maintain Taiwan's identification with China, as well as counter opposition party demands for continuation of an independent Taiwan. Some officials also want to use the visits to the mainland to spread the word about what they regard to be Taiwan's superior economic and political systems.

Since Nov. 2, Taiwan Red Cross officials say, nearly 20,000 people have followed instructions to register with them before leaving for the mainland. Some Taiwan visitors to China no longer know where their relatives live on the mainland and need help from the Red Cross to locate them.

Before the ban was lifted, large numbers of Taiwan citizens already had traveled illegally to China, mostly via Hong Kong, but with the lifting of the ban, the numbers appear to be increasing dramatically.

In China, the Communist Party welcomed the end of the ban but called for much stronger measures toward "reunification" of the two sides, which are technically still at war.

Residents of the People's Republic of China are permitted to travel abroad if they receive a passport from the government and a visa from the foreign country. They are barred from Taiwan, however, by the Taiwanese government.

Under the rules set by Taiwan, anyone other than a person active in the military or civil service can travel to the mainland once a year for the purpose of visiting relatives. The trips are to be made through an intermediate point, such as Hong Kong, and are to last no longer than three months.

Applicants for travel to the mainland leaving the Red Cross office in Taipei are accosted by entrepreneurs selling travel materials designed to prepare the returnees for their trip to China. The salesmen offer guidebooks, maps of the Chinese provinces and video tapes of scenes from China.

One 231-page book entitled, "Answers to all your questions concerning visits to relatives on the mainland," which appeared to contain generally accurate and useful information, warns that because the living standards of Taiwan residents are so much higher than those on the mainland, they should be ready to play "Santa Claus."

"Be prepared psychologically for very big demands," says the book, which is published by a Taipei newspaper.

The guidebook lists "10 big consumer items" that it says are in heavy demand on the mainland. These include cameras, two-door refrigerators, color television sets, automatic washing machines, woolen suits for men and fashionable dresses for women.

A wealthy Taiwan woman who returned recently from her first trip to the mainland in more than four decades reported that she brought a color television set, a small refrigerator, chocolates and gold watches to relatives living near Shanghai.

But her relatives, including her long-lost brother whom she had not seen for 40 years, also asked her for money and were insatiable in their demands. She also said she had to pay more than $100 as a bribe to Shanghai airport police before she could leave the country to return to Taiwan.

Another woman told a foreign journalist that after distributing her gifts, she felt obliged to give away all the clothing that she was not wearing to family members upon leaving the mainland.

But another man who recently returned from a visit to relatives in China's Sichuan Province reported no such problems. He brought no gifts and found his relatives were generous in providing him with food, cigarettes and alcohol.

As for the different standards of living, one Army veteran, 62, who sells fruit in Taipei and just returned from his first visit to the mainland in 38 years, said he became accustomed to the lower living standard on the mainland. But he said he doubted that some of his Taiwan relatives would be able to do so.

The new guidebook also counsels visitors on issues ranging from politics to health. It cautions people to stay away from prostitutes and the foreign currency black market, and says: "Do not touch on sensitive subjects with relatives and friends, especially when there are Communist Party members or officials around, because the concept of what constitutes a state secret is very broad on the mainland," the book says.

"It is very difficult to get dental treatment," the book continues, "There is only one dentist for every 100,000 people on the mainland and there are at least a billion teeth with cavities on the mainland waiting to be treated."