Using kinetic methods it has been possible to show that there are separate forms of UDPglucuronyltransferase (Ec 2.4.1.17) for the synthesis of p-nitrophenylglucuronide and o-aminophenylglucuronide. Thus, p-nitrophenylglucuronide, which is a product inhibitor of its own synthesis, had no effect of the rate of synthesis of o-aminophenylglucuronide; and 4-methylumbelliferylglucuronide and phenylglucuronide were competitive inhibitors of the glucuronidation of o-aminophenol but had no effect on that of p-nitrophenol. Since previous measurements of the rate of glucuronide synthesis as a function of the concentration of UDPglucuronic acid demonstrated that these aglycones did not share a common UDPglucuronic acid binding site, it was concluded that each phenol was metabolized by a separate form of UDPglucuronyltransferase. In order to compare the properties of the two forms of UDPglucuronyltransferases the effect of sulfhydryl group reagents on activities was studied making use of the fact that the p-nitrophenol form of the enzyme is known to have at least three distinctly different sulfhydryl groups. The o-aminophenol form also contained three such groups with nearly identical reactivities as the p-nitrophenol form, indicating close structural similarities between these enzymes. On the other hand, the sulfhydryl groups of the two forms of UDPglucuronyltransferase could be distinguished by their rates of reaction with mersalyl.