1. The ontogenic pattern of xenobiotic carbonyl reducing activity and glucocorticoid 11 beta-oxidoreducing activity of 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) in mouse liver and kidney was examined. In addition, the expression of this enzyme was investigated by means of immunoblot analysis in the same tissues. 2. In liver, the foetus shows low or no enzyme activities. After birth both activities increase dramatically with age and remain then on a high plateau until the time of sexual maturity (4 weeks). After maturity, the enzyme activities decline to intermediate values. The developmental pattern of immunological expression of the liver enzyme corresponds well with that of the enzyme activity. 3. Considerable activities of xenobiotic carbonyl reduction and glucocorticoid 11 beta-oxidoreduction are also present after birth in all developmental stages of the kidney. However, no immunological crossreaction was found in any stages with the antibody against the liver 11 beta-HSD suggesting the presence of a structurally different isozyme form in the kidney. 4. The dramatic increase of both activities during the peri- and postnatal developmental periods suggest a potentially biological significance of the liver 11 beta-HSD isozyme in early animal life. 5. Besides being involved in 11 beta-glucocorticoid metabolism in particular the liver enzyme seems to play an additional role as xenobiotic carbonyl reductase.