Retinoic acid (RA), through nuclear retinoid receptors, regulates the expression of numerous genes. However, little is known of the biochemical mechanisms that regulate RA concentration in vivo. CYP26 (P450RAI), a novel cytochrome P450, is expressed during embryonic development, induced by all-trans RA, and capable of catalyzing the oxidation of [3H]RA to polar retinoids including 4-oxo-RA. Here we report that CYP26 expression in adult liver is regulated by all-trans RA and dietary vitamin A, and is correlated with the metabolism of all-trans RA to polar metabolites. In normal mouse and rat liver, CYP26 mRNA was barely detectable; however, after acute treatment with all-trans RA CYP26 mRNA and RA metabolism by liver microsomes were significantly induced. Aqueous-soluble RA metabolites were detected, but their formation was not induced. The expression of retinoid receptors, RAR-gamma and RXR-alpha, was not changed after RA treatment in vivo. In a model of chronic vitamin A ingestion during aging, CYP26 mRNA expression, determined by Northern blot and RT-PCR analysis, increased progressively with dietary vitamin A (P<0.0001; marginal < control < supplemented) and age (P<0.003). The relative expression of CYP26 mRNA was positively correlated with liver total retinol (log10), ranging from undetectable CYP26 expression at liver retinol concentrations below approximately 20 nmol/g to a three- to fourfold elevation at concentrations >10,000 nmol/g (r=0.90, P<0.0001). We conclude that CYP26 expression and RA metabolism are regulated in adult liver not only acutely by RA administration, as may be relevant to retinoid therapy, but under chronic dietary conditions relevant to vitamin A nutrition in humans.