TOKYO, DEC. 22 -- A Japanese ship left for Antarctica today on a whaling mission that the government described as a research expedition but that environmentalists charged is a ruse to get around an international whaling moratorium.

The Nisshin Maru No. 3, with a crew of 123, left Yokohama in pursuit of 300 minke whales shortly after receiving a final go-ahead. The government of new Prime Minister Noboru Takeshita had delayed the send-off because of concerns that it could aggravate relations with the United States and other countries opposed to whaling. But officials indicated today that the United States had not objected to the whaling expedition.

Japan has said the program is designed to allow scientists to study whales' reproductive capabilities and would not violate the International Whaling Commission moratorium, which allows whaling for research purposes.

The moratorium, in effect since 1985, is to be renewed in 1990. Japan agreed three years ago to go along with the moratorium, beginning this season, after the United States threatened Japanese access to fish in American waters.

But this spring, a week after whaling ships returned from what was supposed to be Japan's last hunt, the government said the boats would be allowed to go out again, but only to kill whales for research purposes. Once the whales were studied, the meat would be sold commercially to restaurants and markets.

Whale meat is popular in Japan, and many Japanese say they simply do not understand the logic or emotions behind campaigns to "save the whales," arguing that the whales they hunt are not in danger of extinction.

In addition, some argue that hunting and eating whales are part of their culture, going back hundreds of years, and they resent foreign intrusion into their customs. Last spring, the government proposed a 10-year research program that would allow whalers to hunt 825 minke whales and 50 of the much larger sperm whales each year. The total annual kill would have been almost half of the 1,941 whales killed by Japanese whalers in their final commercial season last winter.

The research program was voted down by the IWC and bitterly attacked by environmentalists and others as a fraud. Japan then said it would scale back the program and allow the whalers to kill only 300 minkes. Several weeks ago, a majority of an IWC scientific committee indicated that it found no merit in this proposal, either.

This week, members of the environmental group Greenpeace demonstrated by floating in a whale-like raft in front of the lead ship of the whaling expedition. Several of the demonstrators were detained briefly by marine police.

"By reclassifying the hunt as scientific and turning their harpooners into scientists, the Japanese government is using semantic chicanery to continue business as usual for the whaling industry," Greenpeace activist Anne Dingwall said.

Japan has said the research is designed to show that whales reproduce enough to allow hunting.