The effects of a series of fatty acids on the rectal absorption of propranolol (PL) were examined in vitro, using macrogol 1500 base and rat rectal tissue. Lauric acid, at a fatty acid: PL molar ratio of 1:1, produced the largest increase in permeation rate (Js), penetration coefficient and partition coefficient of PL. PL flux was increased 2.5-fold in the presence of lauric acid compared to that without the fatty acid. However, the Js value of PL was decreased at increased molar ratios (e.g., 3:1) of lauric acid. The permeation rate of lauric acid across the rectal membrane was much larger than that of PL. Furthermore, the apparent partition coefficient of PL in an n-octanol/buffer system was significantly increased at a 1:1 molar ratio to lauric acid compared with that of PL alone. These results suggest that a complex-mediated mechanism facilitates PL transport, thereby partially contributing to the enhancement of PL rectal absorption. A similar mechanism is applicable to percutaneous drug absorption, as reported previously. Thus, a portion of PL, after first forming a complex with fatty acids, may rapidly permeate across rectal membranes.