Organophosphate compounds are known to cause a selective increase of β-glucuronidase activity in rat serum. Previous data suggested that increase of serum β-glucuronidase activity was well correlated with decrease of that activity in rat liver microsomal fraction, thereby, suggesting a role of the microsomal enzyme in mediating the organophosphate effect. To investigate further the intracellular sorting pathway of β-glucuronidase in dibuty 1 phosphate-treated rats, liver subcellular fractions were prepared at 12 or 48 h after in vivo administration of [³H]leucine and it was established that microsomal β- glucuron-idase was the origin of the increased serum enzyme. To characterize the intracellular secretory pathway of β-glucuronidase in dibutyl phosphate-treated rats, Golgi subfrac-tions were isolated and a time course study was carried out. At 30 min after administration of dibutyl phosphate, specific activity of β-glucuronidase in GF-2 (Golgi intermediate fraction) and GF-3 (Golgi heavy fraction) was significantly increased to the maximum. Furthermore, colchicine pretreatment of rats caused a delay of the peak of specific activity for 30 min in GF-2 and GF-3, and accumulation of enzyme activity in Golgi subtractions was observed. Colchicine pretreatment also had an inhibitory effect on release of β-glu-curonidase into serum until 30 min after dibutyl phosphate injection. The electrophoretic pattern of microsomal β-glucuronidase on polyacrylamide gel was found to show two major bands of microsomal enzyme type and lysosomal enzyme type in dibutyl phosphate-treated rats. Taken together, these findings indicate that microsomal β-glucuronidase follows the intracellular secretory pathway and is secreted into serum via Golgi complex in response to dibutyl phosphate.