Mature male rats were rendered cryptorchid and followed for up to nine weeks during which serial blood specimens were obtained for multiple hormonal analyses; serial testicular samples were obtained as well. In contrast with control animals, cryptorchid rats showed transient rises in FSH which returned to normal at the end of the study; estrogen levels were high in the final weeks. Plasma testosterone levels were unchanged and LH levels changed little. Light and electron microscopic studies of testicular biopsy specimens showed prompt disruption of spermatogenesis which became more extensive with time. Sertoli cells underwent multiple changes including increased lipid storage and phagocytosis of spermatozoa. Comparisons are made between the sequence of hormonal changes and that of histological changes. In addition, questions are raised concerning the ways in which Sertoli cells are affected by experimental cryptorchidism.