(Purpose) The mechanism for the dose-superlinear increase in the oral bioavailability and double plasma peak phenomenon following oral administration of drugs were investigated.(Methods) The permeability of tributylmethylammonium (TBuMA) across the rat small intestine was measured, and the validity of the proposed hypotheses for the double peak phenomenon was examined.(Results) TBuMA was absorbed in a mixed process of passive diffusion and carrier-mediated effiux,and the saturation of the effiux was responsible for the dose-superlinear increase in the oral bioavailability of TBuMA.The biphasic gastric emptying hypothesis, which has represented the most likely mechanism for the double peak phenomenon, could not successfully explain the double peaks for ranitidine and single peaks for metformin that were observed after the simultaneous administration of the drugs in a solution dosage form.(Conclusion) The dose-nonlinear variation of oral bioavailability should be noted in assessing the oral absorption of drugs.The relevant mechanism(s) for the double peak phenomenon need further elucidation.