Plasma concentrations of cortisol (F) were measured by competitive proteinbinding in plasma samples obtained every 20 minutes for 24 hours from an indwelling venous catheter in 13 normal menstruating females (age 14-53) and 18 normal males (age 14-43). The following parameters were determined: a) 24-hour mean F concentration; b) maximum single F value in any of the 72 samples; c) 8-9 AM F concentration. For women, 24-hour mean F averaged 4.8 µg/100 ml with no change over the age range studied. For men, 24- hour mean F averaged 6.6 µg/100 ml; there was a linear increase with age. Maximum single F values were highly correlated with the 24-hour mean values. 8-9 AM concentrations were markedly lower than the single-stick venipuncture values reported in the literature, by 22% for men and 45% for women. The average for men was 11.6 µg/100 ml and for women 8.5 µg/100 ml. Demonstration of these marked sex differences was only made possible by the multiple sampling technique employed, which eliminates artifacts due to single-stick venipuncture and also takes into account the profound moment-to-moment fluctuation of plasma cortisol concentration due to episodic secretion. These sex differences may have a bearing on the natural history of diseases that show a marked sex difference in incidence, e.g. coronary heart disease (men > women) and autoimmune diseases (women> men).