The pituitary and serum FSH levels were measured in cryptorchid or orchiectomized adult rats at time intervals up to 7 days after surgery utilizing a radioimmunoassay technique. In orchiectomized rats the pituitary levels showed a decline during the first 72 hours after surgery while serum levels showed an increase during this period of time. Between the 3rd and 7th days after surgery the serum levels remained elevated and pituitary levels depressed. In cryptorchid animals the pituitary levels showed marked rise during the first 72 hours after surgery and remained elevated until the end of the experiment. The serum levels showed no change during the first 72 hours and a slight elevation by 7 days. Administration of testosterone propionate (TP) prevented the ochiectomyinduced changes in pituitary and plasma FSH levels. A dose relationship was observed between the serum FSH levels and the dose of TP. With increase in TP dose up to 0.5 mg an increase in pituitary FSH levels was observed. Further increase in TP dose caused a drop in pituitary FSH levels. The plasma levels showed a progressive drop with increasing doses of TP. The data reported in this paper and information available in literature lead the authors to the conclusion that testosterone regulates primarily FSH release rather than its synthesis.