Serum melatonin concentrations from 55 pregnant healthy women (13 during the first, 18 during the second and 24 during the third trimester) and 11 non-pregnant control women were measured radioimmunologically at 11.00 h (Study A). In addition, serum melatonin concentrations from 12 women in early and 11 women in late pregnancy were measured every four hours throughout the day (Study B). Serum melatonin levels during the third trimester of pregnancy (76.5±38.3 pmol/l were significantly (p<0.01) higher than those during the first (29.7±9.9 pmol/l and the second trimester (39.1 ± 11.2 pmol/l and those of non-pregnant control women (41.7± 15.5 pmol/l and there was a positive correlation between the week of gestation and serum melatonin at 11.00 h (Study A). A clear diurnal rhythm in serum melatonin concentrations was found both in early and in late pregnancy (Study B). The amplitude and duration of the nocturnal rise of melatonin were higher during late pregnancy, but there was no clear phase shift. Increased serum concentration of melatonin in late pregnancy may be due to increased synthesis and secretion or retarded metabolism of melatonin during late pregnancy.