The effect of intramuscular injections of PGF2a and PGE2 on concentrations of testos- terone, luteinizing hormone and prolactin were examined in 1-year-old bulls. Jugular venous blood was sampled from indwelling cannulae at frequent intervals before and after injecting O, 5, 15 or 30 mg PGE2 or 30 mg PGF2a in five bulls. Treatments were administered in a Latin-square design. Both PGF2a and 30 mg PGE2 caused increased serum prolcatin concen- trations within 10 min; prolactin concentrations remained elevated longer after PGF2a than after PGE2. Peak prolactin after 15 mg was only half that after 30 mg PGE2 and the change in prolcatin was not significant after 5 mg PGE2. While PGF2a caused significant eleva- tions in serum testosterone and luteinizing hormone (LH), each of these hormones was depressed for 4 to 5 hr by each dose of PGE2. Normal episodic secretion of LH and testoster- one resumed within 8 hr after PGE2 treatment. We conclude PGE2 acutely inhibited release of LH