The catabolic fate of [U-14C]d-glucose has been studied under aerobic conditions of incubation in slices of mature scrotal, immature abdominal and experimentally-induced cryptorchid testes of the rat. In comparison with the mature scrotal testis, CO2 production in the cryptorchid and the immature testis was found to be decreased by 45 and 55%, respectively. In contrast, protein labelling from radio-active glucose was found to be increased in the cryptorchid and the immature testis by a factor of 4·6 and 13·4, respectively. Successive ion-exchange chromatography of the perchloric acid-soluble fraction of the mature scrotal testis resulted in the elution of thirteen peaks of radio-activity from an anionic resin and five peaks of radio-activity from a cationic resin. The major catabolites occurring in slices of the mature scrotal testis were lactate, aspartate, glutamate, glutamine and α-ketoglutarate, accounting for 93% of the total perchloric acid-soluble radio-activity of glucose. Lactic acid accounted for 45·5, 36·9 and 15·9% in the mature scrotal, cryptorchid and immature testis, respectively. An additional unknown peak of radio-activity not found in samples of the mature scrotal testis was eluted from samples of both the immature abdominal and the experimentally-induced cryptorchid testis. It is suggested that of the cells of the seminiferous germinal epithelium, the maturing spermatids are characterized by the highest rate of glucose utilization into acid-soluble catabolites with a marked reduction in glucose catabolism occurring in the spermatogonia and primary spermatocytes.