Since low bioavailability of YJA-20379-8 (3-butyryl-4-[5-R-(+)-methylbenzylamino]-8ethoxy-1,7-naph thy ridine), a new reversible proton pump inhibitor, has been reported after oral administration of the drug to rats, the first-pass organ of the drug was investigated in rats. YJA-20379-8, 50 mg kg(-1), was infused over 1 min via the jugular vein (n=5) or the portal vein (n=5), or was instilled directly into the stomach (n=5) or the duodenum (n=5). After intravenous or intraportal infusion of the drug, the total body clearance of YJA-20379-8 (18.1 and 19.7 mL min(-1) kg(- 1) based on plasma data) was considerably lower than the reported cardiac output (296 mL min(-1) kg(-1) based on blood data) in rats. This data indicated that the first-pass effect of YJA-20379-8 by the lung and heart was negligible. The areas under the plasma concentration-time curve from time zero to time infinity (AUC) after intravenous or intraportal administration of YJA-20379-8 (2760 and 2540 microg min mL(-1)) were not significantly different, indicating that the hepatic first-pass effect of the drug was also negligible in rats. After intragastric or intraduodenal instillation of YJA-20379-8, the extent of absolute oral bioavailability was 18.2 and 33.8%, respectively. Based on gastrointestinal recovery studies, approximately 86.5 and 91.2% of YJA-20379-8 was absorbed from rat gastrointestinal tract after intragastric or intraduodenal instillation, respectively. The data indicated that gastrointestinal and intestinal first-pass effects of YJA-20379-8 were approximately 68% (86.5-18.2) and 57% (91.2-33.8), respectively. The AUC(0-24h) values of YJA-20379-8 were significantly different between intragastric and intraduodenal instillation, indicating that the gastric first-pass effect of the drug was approximately 10% in rats. Therefore, it could be concluded that the low F value of YJA-20379-8 after oral administration of the drug could be due to a considerable (approx. 60%) intestinal first-pass effect in rats.