In 32 pregnant women, we repeatedly determined the levels of alpha-MSH, melatonin, ACTH and progesterone. In the radioimmunoassay, all of these hormones showed a significant increase from the first to the third trimester. In contrast to progesterone, neither the level of alpha-MSH nor that of melatonin rises significantly until between the 28th and 36th week of pregnancy. The authors suggest that the increasing pigmentation may be due to the rising levels of alpha-MSH, while the preferential hyperpigmentation of sexual areas in additionally mediated by sex hormones.