Chronic daily administration of melatonin (MT) can have potent effects on reproduction in the hamster. Various theories have been elaborated to explain these effects but little information has been available on circulating levels of MT following MT administration. We have examined the serum MT response in the male hamster to a single dose of 25 micrograms MT administered in the morning or in the afternoon--the same timing and dose used by others to produce reproductive effects. With both morning and afternoon administration, serum MT increased above 1,000 pg/ml and remained above the highest basal levels during most of the 24-hour cycle. These levels are clearly supraphysiologic ones. The decline in serum MT showed two distinct components following morning administration. Half-life of the initial component which probably represents rapid distribution into tissues was 17.3 min. A half-life of 25.1 h was calculated for the second component. We conclude that use of a 25-micrograms dose of melatonin to study pineal effects may be misleading.