SUMMARY: 1. A surgical technique for the orthotopic transplantation of the ovaries in mice is described and its immediate and long-term difficulties are discussed. 2. The shortened reproductive lifespan of mice which have received such grafts is due primarily to the loss of oocytes during the immediate postoperative period before a new blood supply has developed. Figures are given for the loss, which may extend to between a half and three-quarters of the total complement of oocytes.