To examine diurnal changes in maternal and fetal steroids in chronically catheterized pregnant rhesus monkeys, we have measured cortisol (F), progesterone (P4), estone, and estradiol in maternal blood and F, P4, and dehydroepiandrosterone sulphate (DHAS) in simultaneous samples of fetal blood taken in the morning (0700--1000 h) and in the evening (1700--2100 h). In the mother, the concentrations of F, estradiol, and estrone were higher in the morning than in the evening. In contrast, progesterone was higher in evening than in morning samples. In fetal blood, there was no significant morning-evening difference for F; however, the concentrations of P4 and DHAS were higher in the evening than in the morning samples. In three individual monkeys, the concentrations of P4 and DHAS in serial samples of fetal blood were correlated. We conclude 1) that there are marked diurnal changes in the concentration of several steroids in samples of maternal and fetal blood and 2) that the concentration of F in maternal blood shows the opposite pattern to that of possible fetal adrenal steroids in fetal blood.