In a previous study, it was observed that chlorpromazine (CPZ) inhibited the transport of l-methionine in the rat intestine. The present experiments were designed to delineate the possible mechanism of this effect. Pre-incubation studies with CPZ demonstrated that the tissue concentration of CPZ was much more important than the medium concentration in inhibiting l-methionine transport. This seemed to favor an ‘indirect’ or ‘within-cell’ effect (metabolic inhibition or inhibition of sodium and potassium ion-stimulated adenosine triphosphatase [(Na + + K +)-ATPase]) over a ‘direct’ or ‘surface’ effect (an effect on passive membrane permeability or at the carrier level). CPZ did not affect diffusional entry of l-methionine at the intestinal brush border. The inhibition of non-diffusional mucosal transport of l-methionine by CPZ was not competitive; CPZ inhibited d-galactose transport in the same time-dependent manner as it inhibited l-methionine transport. No CPZ effect on carrier-mediated diffusion of d-xylose was observed. Thus, CPZ effect on active transport processes in the intestine seems to be mediated predominantly within the cell; from the present study it is not clear whether this effect is through inhibition of metabolic energy or inhibition of (Na + + K +)-ATPase.