The drug absorption characteristics of a lipid-containing oral dosage form were studied in human subjects and rats. Diazepam was employed as a model drug and medium-chain triglyceride (MCT) was used as a model lipid. When administered orally to four human subjects, there was no significant difference between diazepam soft capsules and tablets in the average plasma levels of the drug, due to a large intersubject variation. However, when administered to an individual subject repeatedly, the soft capsules showed the tendency of faster drug absorption and superior reproducibility in the plasma level-time curve, suggesting a more uniform drug absorption rate compared with the tablets. The mechanism of this effect was further investigated in the rat model dosing a small amount (2μl/rat) of MCT solution or aqueous suspension. After oral administration, the retention of diazepam in the small intestine was quite small compared with that in the stomach, suggesting the importance of the gastric disappearance rate in drug absorption from these preparations. The gastric drug disappearance rate of MCT solution was faster and significantly less variable than that of the aqueous suspension, which was considered to be the main cause of the more uniform drug absorption rate of the oral diazepam-MCT preparations. It is suggested that diazepam, though it is a weak base, was emptied from the stomach mostly while retained in the lipid, and thus was affected by the movement of MCT in the GI tract.