We have examined whether there are changes in the day-night variation in plasma cortisol in normal women during the third trimester of pregnancy. Maximum cortisol values at 07.30–10.30 h were similar at 30–31, 34–35 and 38–39 weeks of pregnancy. The daily increment in plasma cortisol (fold change) was greater, and the trough values lower at 30–31 than at 38–39 weeks. The time taken for plasma cortisol to fall to 50% of the morning maximum values was longer at 38–39 weeks than at 30–31 weeks of pregnancy. These changes in cortisol rhythms may have implications with respect to alterations in circadian patterns of maternal steroid concentrations as well as fetal biophysical functions.