Six blood samples were taken from each of 33 healthy young men during the course of 2 weeks. From these, the testosterone, dihydrotestosterone, and oestradiol levels were determined. In addition, free testosterone was measured using saliva samples obtained from 23 of the subjects. Both coefficients of variation and variance analyses showed that the inter-individual variability is greater than the intra-individual for all of the hormones. No influence of the day of the week could be ascertained. However, it was discovered that the serum hormone levels dropped with age (weakly significant for oestradiol, insignificant for the androgens) in spite of the subjects being in or near their third decade. The pooled intra-individual correlations between the hormones all fell within the moderate to middle-high positive domain, whereas the inter-individual correlations ranged between highly positive (testosterone to dihydrotestosterone), middle highly positive (free testosterone to total testosterone), and slightly negative (free testosterone to oestradiol).