Plasma 4-en-androstenedione, testosterone, estradiol and estrone were measured during the day in six healthy postmenopausal women and in six breast cancer patients, three of whom received treatment with glucocorticoids. Blood samples were obtained at 8 a.m., 10 a.m., 12 noon, 4 p.m., 8 p.m. and 12 midnight. There was a considerable variation in plasma levels of all steroids during the day; for 4-en-androstenedione the mean within patient coefficient variation was 61.4%, for testosterone it was 28.9%, for estrone it was 17.8% and for estradiol it was 29.2%. While the plasma levels for all steroids tended to be higher in the morning than in the evening, the changes were not statistically significant (Friedman's test: P greater than 0.10). We conclude that although a moderate diurnal variation in the plasma level of these steroids may occur, it is of a moderate magnitude compared to variations due to other causes.