This study on the biliary excretion of sulfadiazine has been established in the rats. 1. Sulfadiazine, administered intravenously to rats with ligated renal pedicles and a cannulated bile duct, rapidly appeared in the bile in high concentration. 2. Between 0-30min. and 30-60 min. after administration, the bile-to-plasma concentration ratios(B/P) of the sulfadiazine were 1. 02-2.67, 1.14-3.79 for 1mg/kg dose, 1.48-3.89, 1.30-3.81 for 10mg/kg, 1.97-4.27, 2.11-4.07 for 50mg/kg, and 1.70-4.21, 1.71-5.34 for 100mg/kg. Thus, B/P ratios at any doses of sulfadiazine greatly exceeded 1.0 at all experimental periods. 3. Furthermore, the biliary excretion of sulfadiazine was inhibited by probenecid significantly. 4. Hepatic clearance of sulfadiazine in the rats was increased from 0.515 to 1.780 ml/60 min. when the dose was raised from 1.0mg/kg to 50.0mg/kg of sulfadiazine, but at 100mg/kg, decreased to 1.250ml/60min. All these results indicate that sulfadiazine is excreted into the bile by active transport process in the rats with ligated renal pedicles and a cannulated bile duct.