1. The transepithelial transport of the beta-adrenoceptor blocking drug, celiprolol, was investigated in monolayers of the well differentiated human intestinal epithelial cell line, Caco-2. 2. The basal-to-apical transport (secretion) of [14C]-celiprolol (50 microM) was 5 times higher than apical-to-basal transport (absorption). In the presence of an excess (5 mM) of unlabelled celiprolol the basal-to-apical transport was reduced by more than 80%, whereas the apical-to-basal transport remained unchanged. 3. Net celiprolol secretion obtained in the concentration range 0.01 to 5 mM displayed saturable kinetics with an apparent Km of 1.00 +/- 0.23 mM and Vmax of 113 +/- 11 pmol/10(6) cells min-1. These results are consistent with saturable active secretion and provide an explanation for the dose-dependent bioavailability of celiprolol. 4. The secretion of celiprolol was sensitive to pH, and decreased in the absence of sodium and in the presence of ouabain, suggesting that transport was coupled to proton and sodium gradients. 5. The secretion of celiprolol was inhibited by substrates for P-glycoprotein (vinblastine, verapamil and nifedipine) and either inhibited or stimulated by typical substrates for the renal organic cation-H+ exchanger (cimetidine, N1-methylnicotinamide, tetraethylammonium and choline), suggesting that there are at least two distinct transport systems. 6. The secretion of celiprolol was also inhibited by other beta-adrenoceptor blocking drugs (acebutolol, atenolol, metoprolol, pafenolol and propranolol) and by the diuretics, acetazolamide, chlorthalidone and hydrochlorothiazide, suggesting that the clinically observed effect of chlorthalidone on the bioavailability of celiprolol occurs at the level of the intestinal epithelium.