Administration of the bovine pineal peptide threonylseryllysine (TSL) to unilaterally ovariectomized (UO) mice as a single intraperitoneal injection on the day of UO reduced the 5-day compensatory ovarian hypertrophy. The same dose of TSL (175 ng/animal) significantly (p < 0.01) reduced serum follicle stimulating hormone (FSH) in UO mice 24 h after surgery and administration of the peptide. In other experiments, adult male rats were anesthetized with urethane and given an intravenous injection of luteinizing hormone releasing hormone (LRH) followed by TSL in doses ranging from 1.5 to 1,500 ng/animal. TSL significantly delayed (p < 0.05) the 15- and 30-min surge of serum FSH observed in control animals after 120 or 180 min in 3 of the 4 doses used (1.5, 15 and 1,500 ng/animal). In the animals receiving 150 ng TSL the LRH-induced FSH surge was delayed (p < 0.05) until 30 min. The results indicate that the pineal tripeptide may inhibit or modify FSH secretion in the mouse and rat.