Piperine (1-peperoyl piperidine), a major component of the Piper species was reported recently by us to inhibit the activities of rat hepatic monooxygenases and UDP-glucuronyltransferase. This study explores further the basis of inhibition of glucuronidation. The effect of piperine on the rate of glucuronidation of 3-hydroxybenzo(a) pyrene and UDP-glucuronic acid content in the intact isolated epithelial cells of the guinea-pig small intestine was studied. The cells offered a fairly good system to study the modulation of glucuronidation activity. Glucuronidation of 3-hydroxybenzo(a) pyrene was dependent on the time of incubation, cellular protein and substrate concentration. From the kinetics of glucuronidation of 3-hydroxybenzo(a)pyrene in the isolated cell preparation the Vmax of 0.5 nmol of BP-3-glucuronide formed per min/mg of protein and Km of 25 microM were observed. The endogeneous concentration of UDP-glucuronic acid observed was 1.6 to 2.3 nmol/mg of cellular protein. Piperine caused a concentration-related decrease in UDP-glucuronic acid content and the rate of glucuronidation in the cells. It required much lower concentrations of piperine than D-galactosamine to diminish the endogeneous level of UDP-glucuronic acid. Rate of glucuronidation of 3-hydroxybenzo (a) pyrene was dependent on the endogeneous level of UDP-glucuronic acid. At 50 microM piperine, the rate of glucuronidation was reduced to about 50% of the basal rate. Piperine caused noncompetitive inhibition of hepatic microsomal UDP-glucuronyltransferase with Ki of 70 microM. The studies demonstrate that piperine modifies the rate of glucuronidation by lowering the endogeneous UDP-glucuronic acid content and also by inhibiting the transferase activity.