1. Cutaneous UDP-glucuronosyltransferase activity (E.C.2.4.1.17) was demonstrated in rat- and hairless mouse-skin microsomes using 1-naphthol as substrate. 2. Addition of the detergent Brij 35 increased the activity by approximately twofold in both species. 3. Inhibitor studies demonstrated that under the assay conditions used any UDP-glucuronic acid pyrophosphatase or beta-glucuronidase present did not interfere with the conjugation reaction. 4. Substrate inhibition was observed in hairless mouse-skin preparations and biphasic response to increasing naphthol concentration was seen in rat-skin microsomes. 5. The apparent Km values were considerably lower than those reported for liver. The sp. activity (per mg microsomal protein) in unactivated rat-skin microsomes was about 50% of that reported in unactivated rat-liver microsomes. 6. Pretreatment with 3-methylcholanthrene resulted in a small increase in cutaneous UDP-glucuronosyltransferase activities in both species.