Background: Inhibition of the Na+/H+ exchanger (NHE) is cardioprotective, but dosage and timing of NHE-inhibitors are critical for their efficacy. We studied the effect of a new dosing regime of the NHE-inhibitor cariporide on myocardial function and damage after cardioplegic arrest (CPA) and determined its myocardial and serum concentrations. Methods: 3 pigs received a bolus of 180 mg cariporide intravenously (i. v.) and were sacrified shortly thereafter to allow measurement of the myocardial concentrations of cariporide. Subsequently, 10 pigs were randomized to receive either i.v. cariporide (bolus followed by an infusion of 40mg/h) or placebo. Cardiopulmonary bypass was initiated, and the heart was arrested for 60 minutes by infusion of St. Thomas Hospital solution. Left ventricular (LV) function was studied using microsonometry. Myocardial damage was assessed by troponin T. Serum concentrations of cariporide were measured throughout the study, and myocardial concentrations were measured before the end of CPA and 180 minutes thereafter. Results: Cariporide was present in all myocardial specimens (median: 1.4 ng/mg) studied priorly. In the main study, LV function or myocardial damage did not differ significantly between the groups at any time point. Stable serum cariporide concentrations were achieved (3.4±0.5 μg/ml). Cariporide was detectable in only one of the myocardial biopsies obtained before the end of CPA, but 180 minutes thereafter, the myocardial cariporide concentration was 2.5±0.3 ng/mg. Conclusion: We observed no effect of i. v. cariporide on LV function or myocardial damage after cardioplegic arrest. Our data suggest that cariporide is washed out of the myocardium by repeated application of crystalloid cardioplegia. Thus, the mode of delivery also appears to be critical for cardioprotection with NHE-inhibitors.