1. Interspecies differences in the intestinal hydrolysis of glucuronide conjugates (phenolphthalein glucuronide, 4-methylumbelliferone glucuronide, morphine-3-glucuronide) were evaluated in mouse, rat and rabbit small intestine and caecum. 2. beta-Glucuronidase activity in the caecum was 50-200-fold higher than in the small intestine for all species and substrates studied. 3. There was evidence of a similarity between species in the capacity of the gut contents cultures from the proximal and distal small intestine to hydrolyse phenolphthalein glucuronide and 4-methylumbelliferone glucuronide. 4. Morphine was not liberated from morphine-3-glucuronide in detectable amounts in the proximal and distal small intestine. 5. Species- and substrate-specific differences were identified in the capacity of the caecal microbiota to hydrolyse the glucuronide conjugates studied. 6. The capacity of the rabbit caecal microbiota to hydrolyse all three glucuronides was significantly lower than those of both rat and mouse. 7. Morphine was metabolized to codeine in low, but detectable levels in all three species.