Orchiopexy was performed to 12 unilateral cryptorchid adult, 3 unilateral cryptorchid young, and one bilateral cryptorchid adult dogs. The testicular sizes of these dogs were measured, biopsy performed on the testes and peripheral and spermatic venous blood samples were collected at intervals of 2, 4 or 8 weeks after the operation. Biopsy tissue specimens were examined after staining with PAS-hematoxylin. Blood plasma testosterone (T) levels were measured by radioimmunoassay. Semen was collected by digital manipulation every week. In all the dogs the cryptorchid testicular size increased after orchiopexy. Neither spermatids nor spermatozoa were observed in any cryptorchid testes at the time of operation. Spermatozoa first appeared found in the seminiferous tubules in the young and adult cryptorchid dogs 8 and 10 weeks after the operation, respectively. In these dogs the histological findings of the cryptorchid testis were the same as those of the scrotal testis 20 weeks after orchiopexy. There was an increase in the number of germ cells in the seminiferous tubules of the scrotal testis in these dogs after the operation. Though the semen became worse temporarily in quality in the adult cryptorchid dogs, it recovered in the young cryptorchid dogs some weeks after orchiopexy. Plasma T levels increased gradually both in peripheral blood and in bilateral testicular venous blood after orchiopexy.