This study assessed the process of gastrointestinal drug absorption in vivo using PET. Methods: ¹⁸F-FDG was used as a model probe and was orally administered to rats as a solution. PET scans were obtained of the whole body and abdominal region under conscious and anesthetized conditions. Blood samples were routinely taken from the femoral vein during scanning. The rate of gastric emptying and intestinal absorption of ¹⁸F-FDG was estimated from the time profiles of radioactivity in the stomach and small intestine. In addition, nonradiolabeled 2-fluoro-2-deoxy-d-glucose (2-FDG) was used in an intestinal closed-loop experiment to compare the intestinal permeability of 2-FDG with that of d-glucose. Results: In conscious rats, gastrointestinal absorption of ¹⁸F-FDG was rate-limited by the process of intestinal membrane permeation, because the permeability of 2-FDG through the intestinal membrane was low compared with that of d-glucose. In anesthetized rats, the gastric emptying rate of ¹⁸F-FDG decreased dramatically whereas the intestinal absorption rate constant was not significantly different from that in conscious rats. As a result, the rate-limiting step of gastrointestinal absorption of ¹⁸F-FDG was shifted to the gastric emptying process by anesthesia. Conclusion: PET technology is a powerful tool for in vivo analysis of the gastrointestinal absorption of orally administered drugs.