11β-hydroxysteroid dehydrogenase (11β-HSD) in Leydig cells regulates sterodogenesis by controlling intra cellular glucocorticoid (corticosterone, B, in rat) concentration.Prior to the 26th postnatal day, 11β-HSD is absent from rat immature Leydig cells. Asthe Leydig cells are matured, the enzyme is gradually produced. The highest levels of11β-HSD activity are present in adult rat Leydig cells. 11β-HSD controls the intracellular glucocorticoid concentration in Leydig cells and the glucocorticoids at the physiologicallevels also regulate levels of 11β-HSD activity in Leydig cells. The expressions of 11βHSD mRNA in Leydig cells from three different age groups of rats and adrenalectomizedrats (ADX), with and without B replacement were Observed in this study. The steady statelevels of 11β-HSD mRNA could not be detected in Leydig cells from immature rats aged21 days, but this could be detected in those aged 45 days. The highest levels of expressionOf 11β-HSD mRNA were found in adult Leydig cells. The mRNA expression of 11β-HSD was declined in Leydig cells after adrenalectomy, and this decline was prevented byB replacement (the levels were restored to control). The results indicated that 11β-HSDmRNA leVels expressed in three different age groups of rats are parallel with those ofantigen by immunohistochemical analysis and with those of enzyme activity by biochemicalmeasurement in previous studies. Similarly, the effect of B at physiological and endogenous levels on the expressions Of 11β-HSD mRNA corresponded to that on enzyme activity. 11β-hydroxysteroid dehydrogenase (11β-HSD) in Leydig cells regulates sterodogenesis by controlling intracellular glucocorticoid (corticosterone, B, in rat) concentration. Prior to the 26th postnatal day, 11β-HSD is absent from rat immature Leydig cells. Asthe Leydig cells The highest levels of 11β-HSD activity are present in adult rat Leydig cells. 11β-HSD controls the intracellular glucocorticoid concentration in Leydig cells and the glucocorticoids at the physiological levels also regulate the regulation of 11β-HSD activity in Leydig cells. The expressions of 11β HSD mRNA in three different age groups of rats and adrenalectomized rats (ADX), with and without B replacement were observed in this study. The steady state levels of 11β-HSD mRNA could not be detected in Leydig cells from immature rats aged21 days, but this could be detected in those aged 45 days. The highest levels of expressionOf 11β-HSD mRNA were found in adult Leydig cells. The mRNA expression of 11β-HSD was declined in Leydig cells after adrenalectomy, and this decline was prevented by B replacement (the levels were restored to control). The results indicated that 11β-HSD mRNA leVels expressed in three different age groups of rats are parallel with those of antigen by immunohistochemical analysis and with those of enzyme activity by biochemical measurement in previous studies. Similarly, the effect of B at physiological and endogenous levels on the expressions Of 11β-HSD mRNA corresponded to that on enzyme activity.