BALTIMORE, SEPT. 20 -- The twins born joined at the back of their heads and separated by surgeons two weeks ago were back on ventilators today because their lungs had partially collapsed, a hospital spokeswoman said.

"The twins are breathing well with ventilator support," said Lisa Hillman, a spokeswoman for Johns Hopkins Hospital's Children's Center. "Doctors anticipate they will remain on the ventilators for at least a week."

The 7-month-old twins had been taken off the machines Friday, she said.

Hillman said that doctors did not know what caused the partial lung collapse but it was considered a possible complication of the 22-hour operation that separated Patrick and Benjamin Binder, from Ulm, West Germany.

Both boys were in critical but stable condition, she said.

Patrick was put back on the breathing apparatus late Saturday and Benjamin was placed on the ventilator this morning, Hillman said.

On Friday, Benjamin was returned to surgery for a second skin graft to cover the area where he and his brother had been joined.

During the 6 1/2-hour procedure, doctors removed skin and muscle from his back and applied it to the back of his head, Hillman said. The new grafts replaced those made earlier last week, which were not working, Hillman said.

The twins remain sedated, but are out of a prolonged anesthetic coma induced after the operation to allow their brains to recover from the stress of the surgery, Hillman said.

Both are responsive to their parents, open their eyes at their mother's touch and have moved their arms and legs, Hillman said.