Diurnal variations in plasma unconjugated and total estriol were assessed in 11 third-trimester subjects with uncomplicated pregnancies. Commercially available 125I-IabeIed radioimmunoassay kits were used. Total plasma estriols reach a nadir during the hours of sleep (400 and 700 hours) which exceeds the episodic fluctuations seen from day to day or during a given 90-minute period. Plasma unconjugated estriol fluctuations over 24 hours did not significantly exceed our previously reported episodic fluctuation of 15.6 ± 8.2%. The data are interpreted as showing no cir-cadian rhythm, but reflecting, in the case of total plasma estriols, an effect of improved renal clearance during hours of rest. Plasma unconjugated estriol emerges as the test of choice in the monitoring of high-risk pregnancies.