President Reagan will undergo surgery in Washington Saturday for a long-standing and worsening condition that causes the ring finger of his left hand to bend inward and resist straightening.

The president will enter Walter Reed Army Hospital on Friday and is expected to return to the White House on Sunday, Dr. John Hutton, the White House physician, announced Tuesday.Hutton said the president will be operated on for a disease known as Dupuytren's contracture.

"This procedure should not interrupt his normal schedule, other than the obvious presence of a dressing on the left hand," the doctor said.

"The president's contracture was minimal but present when he took office in 1981, and has been followed over the past eight years," he said. "Within the past year, it has progressed to the degree that surgical therapy is now necessary."

He said the surgery is expected to take two to three hours. The president's arm will be put to sleep, but no general anaesthetic is planned, he said.

Dr. Charles Lane, a hand specialist in private practice in Beverly Hills, said the cause of Dupuytren's contracture is not known.