The serum and amniotic fluid concentrations of melatonin (MT) were measured by RIA during human labor in different conditions related to the type of delivery and the time of the day of delivery. Serum MT concentrations displayed a normal diurnal rhythm, resembling that of nongravid women; the mean concentration [163.8 +/- 149.6 (+/- SD) pmol/L] at night was significantly (P less than 0.001) higher than that during the day (31.4 +/- 16.3 pmol/L). The amniotic fluid MT concentration, which showed a significant positive correlation to the serum MT concentration (r = 0.625; P less than 0.01), also showed an obvious diurnal rhythm; the mean MT concentration was significantly (P less than 0.01) higher during the night [99.3 +/- 59.3 (+/- SD) pmol/L] than during the day (58.9 +/- 33.5 pmol/L). Reverse phase high pressure liquid chromatography confirmed that the amniotic fluid MT immunoreactivity eluated as synthetic MT. During the night the mean amniotic fluid MT concentration [99.3 +/- 59.3 (+/- SD) pmol/L] was significantly (P less than 0.01) lower than that in serum (178.9 +/- 189.6 pmol/L). The progress of delivery, estimated by cervical dilatation, did not affect serum MT concentrations. Induction of delivery by amniotomy and/or oxytocin, and operative delivery by cesarean section had no effect on serum MT concentrations. Human MT secretion does not seem to be influenced by the physical stress of labor or endocrine changes associated with parturition. The single factor regulating MT secretion during human delivery appears to be the time of day.