A single injection of 300 mug oestradiol benzoate (OEB) or 1.25 mg testosterone propionate (TP) on day 1 of life led to significant changes in the activity of enzymes involved in steroid hormone metabolism in kidney, adrenal and gonadal tissues of adult rats. In the kidney, the enzyme activities of male rats reacted to OEB, but not TP, by the development of normal female levels. With one exception the enzyme activities of the kidney of female rats did not respond to either steroid. In the adrenal of both sexes 5alpha-reductase reacted to OEB, but not TP treatment, by a fourfold increase in activity. In the ovary all the enzymes investigated responded both to OEB and TP treatment by a fall in activity; 20alpha-hydroxysteroid dehydrogenase activity fell to undetectable levels. In the testis, OEB and TP treatment led to contrasting effects. With the exception of 5alpha-reductase all the enzymes tested in this organ responded to OEB by a rise in activity. Where TP had any effect, it produced a slight decrease in activity.