About 300 adult male lizards, Anolis carolinensis, measuring over 59 mm in snout-vent length were used in studying the role of the pars distalis in controlling testicular function. Both maintenance and development of the germ cells and androgen production depend on the adenohypophysis. Rapid regression in testicular size and spermatogenic activity follow removal of the pars distalis in animals kept at 31 C. Germ cells begin degenerating within 2 days, and all except a few spermatogonia are lost within 10 days. Regression is complete within 14 days; enlarged testes decrease from 35 to 2 mg. Autoradiographic studies demonstrate that in intact animals spermatogonia develop to the stage of transforming spermatids within 14 days; thymidine uptake by germ cells ceases within 4 days after hypophysectomy.