The acid and enzymatic hydrolysis rates for two morphine metabolites, morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G), were compared. Urine fortified with M3G or M6G was hydrolyzed in a boiling water bath or with beta-glucuronidases. Optimal temperatures for beta-glucuronidases were 60 degrees C for Helix pomatia and abalone and 50 degrees C for Escherichia coli. A 3-h incubation with 1,000-1,500 Fishman units of enzyme gave complete hydrolysis of M3G. The enzymatic hydrolysis rate of M6G was approximately 25% of the hydrolysis rate of M3G and required 24 h for completion. The acid hydrolysis rate of M6G was slower than M3G and required a 0.5-h incubation with a 1:1 mixture of concentrated HCl in a boiling water bath for completion. For comparison, the acid hydrolysis rate of codeine conjugates was found to be similar to the rate of M6G, but the enzymatic rate of codeine conjugates appeared slower than the rate of M6G.