The production rate (PR) values of delta 4-androstenedione (A), testosterone (T), oestrone (E1) and oestradiol (E2) have been determined in a large group of post-menopausal women following constant infusions of radiolabelled hormones and radioimmunoassay of endogenous steroid concentration. The mean +/- SE age was 64 +/- 2 yr, ranging from 46 to 91 yr and the mean +/- SE weight was 144 +/- 4 lb. When the PR values were related to age by linear regression analysis no significant correlation could be found for PRA, PRT or PRE1 and the age of the subjects. There was, however, a significant correlation between PRE2 and age. There was a significant correlation between the PR values for each of the four steroids and the weights and body surface areas of the subjects. In addition, PRA correlated directly with both PRT and PRE1 in these subjects in which both PR values were measured. The PR values for each steroid were significantly smaller in the post-menopausal women compared to the mean PR values of a large group of pre-menopausal women. We conclude that age, per se, does not appear to influence the PR values for A, T and E1 but does for E2. The subjects weight, however, has a major influence for the PR values of all four steroids.