Diurnal changes in peripheral plasma concentrations of melatonin (MLT) and effects of 2 hr light exposure during the nighttime and 2 hr dark exposure during the daytime on MLT concentrations in goats were examined in natural lighting condition. Two breeds of goats, the Saanen goat as a seasonal breeder at the breeding season, and the Shiba goat as a non-seasonal one, were used while they were at luteal phase. Plasma concentrations of MLT showed a marked diurnal change in both breeds of goats, which was high during the nighttime (20-120 pg/ml) and low during the daytime (<20 pg/ml). Light exposure during the nighttime (for 2 hr) caused an abrupt decline in plasma concentrations of MLT to the basal level; and when the light was turned off, plasma MLT began to increase. On the contrary, 2 hr dark treatment during the daytime did not induced a rapid increase in plasma MLT as observed at the onset of the night. The pattern and the magnitude of the nocturnal elevation in plasma MLT and the responses to the dark or light interruption were virtually identical between the two breeds. These results suggest that the secretion of MLT in goats is driven by the endogenous rhythm, though the lighting stimulus temporarily suppresses the MLT secretion.