A transdermal delivery device (TDD)1 was applied to four human subjects to investigate whether melatonin (MT) could penetrate through human skin. The TDD (total surface area of 3.80 cm2) was applied to the forearm of each subject. Plasma MT concentrations increased above baseline in approximately 2-4 hours, although steady state was not achieved in the 8-hour study period. Intersubject variation of plasma MT among four subjects was noted. Urinary excretion of 6-sulphatoxymelatonin (6-STMT), a major metabolite of MT in humans, increased as plasma MT concentrations increased. Cumulative amounts of urinary 6-STMT increased over a 6-hour period when the TDD was applied and were three times greater than in controls. The urinary excretion rate of 6-STMT was statistically correlated with plasma MT concentration among subjects (r2 = 0.77). These data suggest that the urinary excretion rate of 6-STMT can be used as an index of MT plasma concentrations in human subjects. Although an intersubject variability in both plasma MT concentration and urinary excretion rate of 6-STMT was noted, it was evident that MT can be delivered transdermally in human subjects.