Blood samples were obtained from a group of regularly-cycling normal women (n = 13) and a group of normal men (n = 5) at 15 min intervals for periods ranging from 9-14 h whilst the subjects remained resting quietly in bed. The concentration of plasma oestradiol was measured in a radioimmunoassay which had been carefully optimised and the mean concentration and coefficient of variation calculated for each subject. Plasma oestradiol concentrations were found to fluctuate rapidly in all subjects. The largest sample-to-sample variation was found in regularly cycling women near mid-cycle, and the least variation occurred in the male subjects. The coefficients of variation were approximately twice the intra-assay variation confirming that the observed pulsatile patterns of secretion were not due to technical errors. An analysis of variance performed on the mean hourly concentrations from each subject showed a significant variation (P less than 0.001) between sampling periods with peaks occurring during the first hour of sampling (08.00-09.00 hours). This result is suggestive of the presence of a circadian rhythm although it is not conslusive.