The main elimination site of organic cations (OC) is located in the proximal tubule (PT) of the kidney. This transport of OC involves recycling of endogenous OC (e.g. dopamine) as well as excretion of exogenous metabolites and xenobiotics. This study aims at elucidating the properties and regulatory mechanisms of OC transport in isolated mouse PT segments as this knowledge is increasingly interesting when studying transgenic mouse models. OC transport across the basolateral membrane of freshly isolated and collapsed PT segments (S2/S3) of male C57/BL6 mice (Charles River, Germany) was measured as initial uptake rate of the fluorescent dye 4-(4-dimethylamino)styryl-N-mehtylpyridinium (ASP, 1mM) in a microtiter plate fluorescence reader. Expression of OCT1 and 2 is similar in S2 and S3 segments. Over all expression of OCT 1 is app. 1.3 of OCT 2 and app. 100 times of OCT 3. Distribution of OCT1 is similar in all 3 segments, that of OCT2 is app. 30 % higher in S2/3 compared to S1. ASP uptake was inhibited by TEA with an IC50 of 14mM (n=4 to 28), which compares to our published values obtained in isolated human PT (63mM) or rat OCT1 (58mM) or rat OCT2 (176mM) expressed in HEK293 cells. Inhibition of the Ca2+/calmodulin complex, by calmidazolium (5mM) reduced ASP-uptake by 33 ± 11 %, n=24 comparable to findings for the rat and human OCTs expressed in HEK293 cells. AngiotensinII (100nM) had no significant effect on ASP uptake. The p56lck tyrosine kinase inhibitor aminogenistein (10mM, n= 50) inhibited ASP uptake by 46 ± 10 %. OC transport in freshly isolated mouse PT shows similar substrate specificity for ASP and TEA compared to that observed for specific rat OCT isoforms. Also its regulation by Ca2+/calmodulin and p56lck tyrosine kinase demonstrates qualitative comparablities. OCT 1 and 2 are the relevant isoforms in mouse PT. Supported by a grant of the German Research Council (CI 107/4-1)