BALTIMORE, SEPT. 5 -- A 70-member surgical team at Johns Hopkins Hospital began work this morning to separate 7-month-old Siamese twins, joined at the head, in a complicated operation that doctors say has a 50 percent chance of success.

The operation on the West German twins began at 7 a.m. and was expected to continue until Sunday morning, said Lisa Hillman, a spokeswoman for the hospital's Children's Center, where the surgery is being performed.

Hopkins officials said they will release medical details after the operation.

Patrick and Benjamin Binder of Ulm, West Germany, were born connected at the back of the head and share a major vein, but their brains are separate.

They were admitted to Johns Hopkins Wednesday after arriving in the United States last Satur- day.

Siamese twins occur in about 1 per 2 million to 2.5 million births. The survival rate of twins who have been separated is normally no greater than 50 percent, said Dr. Benjamin Carson.

Carson, the hospital's director of pediatric neurosurgery and one of 20 doctors involved in the operation, added that the rate is hard to determine because of a lack of statistics.

The surgical team had practiced the separation procedure using the most lifelike dolls they could find, Carson said.

A West German magazine, Bunte, paid the parents of the twins an undisclosed amount so that only its staff could interview or photograph the family.

The National Enquirer purchased North American rights to the twins' story from Burda Publications, the company that owns Bunte.