1. Phase I and phase II biotransformation was compared in streptozotocin-induced hypoinsulinemic (STZ) and genetic hyperinsulinemic (WKY-fatty) rats. 2. Total cytochrome P-450 concentrations were reduced in both STZ and WKY, whereas styrene oxide hydrolase and benzphetamine N-demethylase activities were normal in STZ and reduced in WKY. 3. UDP-glucuronosyltransferase activity was decreased toward testosterone and 1-naphthol in STZ and WKY, and was increased toward estrone in the obese female WKY. 4. Glutathione S-transferase activity was decreased in STZ toward 1-chloro-2,4-dinitrobenzene, ethacrynic acid and sulfobromophthalein, but was similar to that in normal rats for WKY.