Recent studies have shown that granisetron incurs glucuronide conjugation in the rat. This metabolic reaction may facilitate biliary excretion and reabsorption of the drug through enterohepatic circulation. This study was designed to evaluate the biliary excretion of granisetron employing a specially designed microdialysis shunt probe to monitor biliary drug concentrations without fluid losses. To avoid obstruction of the bile duct or bile salt waste, the shunt linear probe connected the bile duct between liver and duodenum. The utility of this microdialysis design was demonstrated by studying the hepatic metabolism and biliary excretion of granisetron in the rats. Following intravenous administration (3 mg/kg), granisetron was measurable in the bile microdialysate. Samples were eluted with a mobile phase containing 25 mM acetate buffer (pH 5.3): acetonitrile (78:22, v/v). The 25 mM acetate buffer was prepared with 25 mM sodium acetate with pH adjusted to 5.3 by glacial acetic acid. It was concluded that the in vivo microdialysis technique yielded useful data on the biliary excretion of granisetron.