Studies were undertaken to determine the effects of intravenous infusions of bovine testicular extracts on plasma levels of FSH and LH in castrate male sheep. Decapsulated bovine testes were homogenized with phosphate buffer, the homogenates centrifuged at 30,000 X g for 30 min, and the supernatants extracted with diethyl ether. The aqueous extracts were filtered using Diaflo XM-100 membranes and the ultra-filtrates lyophilized. The extracts were infused over 24 hours, and plasma FSH and LH were measured prior to, during, and after the infusion. Control studies using infusions of saline and liver extracts demonstrated no decrease in FSH levels in 6 out of 7 controls, while LH levels tended to rise. Following infusions with the testicular extracts, the levels of FSH were selectively decreased to between 42% and 85% of baseline levels, and they subsequently remained suppressed for at least 24 hours. These results suggest that the testis is the site of production of a water-soluble, non-steroidal substance which exerts an inhibitory effect on the production and/or release of FSH.