Blood samples were obtained from adult female Weddell seals in McMurdo Sound, Antarctica, utilizing four different seals every 2 h for 24 h on two occasions in November 1987. Plasma levels of melatonin were mostly near zero, whereas the values for cortisol were high (2101 +/- 54 and 1908 +/- 108 (S.E.M.) nmol/l, n = 52). For both hormones there was no evidence of a diurnal pattern of secretion. Juvenile seals sampled in December 1985 also had no evidence of a diurnal secretion pattern of melatonin, but blood levels of melatonin were higher than in adults. The results indicate that secretion of melatonin in adult Weddell seals ceases during the Antarctic summer.