SUMMARY Sexually immature male ferrets received electrolytic lesions in the rostral mediobasal hypothalamus, and another group of males received sham operations. Testicular size, as estimated by weekly palpation, increased rapidly after the placement of lesions. Four weeks after the operation the right testis and epididymis were found to weigh significantly more in lesioned than in sham-operated animals, whereas body weight was the same in the two groups. The right testis of lesioned ferrets had a significantly higher Leydig cell index as well as significantly larger luminal and outer diameters of the seminiferous tubules. In addition, histological examination and classification of the seminiferous tubules showed that spermatogenesis was advanced in lesioned ferrets, with pachytene spermatocytes being the most advanced germ-cell type present in a significantly higher percentage of tubules, and spermatogonia being the most advanced germ-cell type present in a significantly lower percentage of tubules. Finally, at this time the concentration of testosterone per testis was significantly higher in lesioned than in sham-operated ferrets. When the remaining testis was biopsied 20 weeks after the operation it was found that spermatogenesis was still more advanced and the concentration of testosterone in blood was significantly higher in lesioned ferrets. By the time autopsy was performed 38 weeks after the operation, all parameters of testicular function in the sham-operated male ferrets had caught up with those of lesioned animals. In a second experiment it was found that hypothalamic lesions accelerated testicular growth without interfering with the subsequent occurrence of an annual regression and recrudescence of the testes.