The in vivo metabolism of 9-cis-retinoic acid (9-c-RA), an endogenous ligand of retinoid X receptors (RXRs), which can also bind to retinoic acid receptors (RARs), was examined in pregnant mice and rats following a single oral dose of 100 mg 9-cis-retinaldehyde (9-c-RAL)/kg body weight. 9-Cis-retinoyl-beta-D-glucuronide (9-c-RAG), a metabolite not found in vivo before, was a major metabolite of 9-c-RA in mouse plasma and was also present in all mouse tissues examined as well as in rat plasma. In both species putative oxidation products of retinoic acids and high levels of retinyl esters were found. Concentrations of retinoic acid isomers and retinoyl-beta-D-glucuronides in the mouse plasma greatly exceeded those of the rat plasma. The finding of high levels of 9-c-RAG underlines the importance of glucuronidation in the metabolism of retinoids.