Basolateral uptake of organic cations in hepatocytes is mediated by hOCT1 and in proximal tubules by hOCT2. The transport of cationic (ß-blockers pindolol and atenolol) and zwitterionic (antibiotic quinolones ofloxacin and norfloxacin) drugs by hOCT1 and hOCT2 was studied in stably transfected HEK293 cells. Transport was examined fluorimetrically with the cation 4-(4-(dimethyl-amino)styril)-methylpyridinium iodide (ASP, 1μM) as substrate. Drug uptake by OCTs was evaluated by comparing their concentration in lysates of hOCT1, hOCT2 and non transfected cells after 10 min incubation at 37°C and 4°C with 100 μM ß-blockers or 500 μM quinolones, respectively. Drug concentrations in lysates was evaluated by mass spectrometry. ASP-uptake by hOCT1 was inhibited concentration-dependently by pindolol (EC50=127 μM), atenolol (EC50=3.2 mM), ofloxacin (EC50=3.8 mM) and norfloxacin (EC50=1.4 mM). hOCT2 showed a similar apparent affinity for pindolol (EC50=145 μM) and norfloxacin (986 μM), but higher values for atenolol (EC50=93 μM) and ofloxacin (EC50=693 μM). Atenolol, pindolol and ofloxacin were specifically accumulated in the cells by hOCT2, but not hOCT1. Renal excretion is therefore the principal elimination route for atenolol, pindolol and ofloxacin and comedication will lead to mutual interaction via hOCT2-mediated renal secretion. However, also the inhibition of hepatic organic cation transport can have important pharmacotherapic implication in case of comedication with other cationic drugs. Supported by the IMF (CI 1 2 04 37)