A cDNA was isolated from a library prepared with 15-day-old rat liver RNA and was shown to correspond to CYP2C23 with some minor alterations which modified the open reading frame. The expression of CYP2C23 was examined in rat liver, kidney, lung and testis and during ontogenesis. An appreciable amount of CYP2C23 RNA was observed in liver and kidney but the age-dependent expression was quite different between these two tissues. In liver, expression reached its maximal value in early neonates and remained quite stable, while the increase was progressive in kidney before declining after 3 weeks of age. In the liver, classical inducers of cytochrome P-450 decreased the CYP2C23 RNA content. Experimental data confirm the classification of CYP2C23 as a constitutive member of the 2C subfamily and clearly establish its age- and tissue-dependent expression in rat.