The activities of the melatonin-forming enzymes hydroxyindole-O-methyl transferase (HIOMT) and N-acetyl transferase (NAT) were determined in pineals of mature female Coturnix quail receiving implants of melatonin (10 µg, 1 µg, 0.1 µg) weekly for 4 weeks. Both HIOMT and NAT were significantly decreased by implants of 10 and 1 µg melatonin compared to vehicle and intact controls. Oviductal weights tended to be higher with increasing quantities of melatonin. These results suggest that melatonin exerts a feedback inhibition upon its synthesizing enzymes which may be of particular importance during the rapid sexual maturation of this spec