The effect of carbachol on the absorptions of caffeine, aminopyrine and salicylic acid in the rat small intestine was studied, using an in situ recirculating perfusion method. Carbachol, either added to the perfusate or injected intravenously, remarkably reduced drug absorption. Upon the addition of carbachol, the perfusate volume of intestine decreased markedly and there was a positive correlation between the perfusate volume and the drug absorption rate. No physicochemical interaction, as determined by measurement of the partition coefficient between isoamyl acetate or benzene and phosphate buffer (pH 7.0), was noted between the drugs and carbachol in solution. Our results suggest that the carbachol-induced decrease in drug absorption is due to a reduction in the perfusate volume in the intestine upon recirculating perfusion.