In cardiac muscle, the electrogenic Na-Ca exchanger plays important roles in determining action potential shape and in the beat-to-beat homeostasis of intracellular calcium. In this study we tested the actions of a putative cell-permeant blocker of the cardiac sarcolemmal Na-Ca exchange, ”Myristyl- (Myr-) FRCRCFa”. Experiments were performed using isolated rabbit right ventricular myocytes and whole-cell patch-clamp at 35–37°C. The Na-Ca exchange current (I Na-Ca), L-type calcium current (I Ca,L), inward rectifier potassium current (I K1) and delayed rectifier potassium current (I K) were compared in untreated cells and cells incubated in a solution containing N-myristylated FRCRCFa. With other major currents blocked, I Na-Ca was measured as the Ni-sensitive component of current during a voltage ramp applied from the holding potential of –40 mV, between +80 and –120 mV (ramp velocity 0.1 V s–1). In untreated cells, I Na-Ca at +60 mV was 7.1±0.6 pA/pF and at –100 mV was –2.7±0.3 pA/pF (n=9). After a 15-min pre-incubation with 20 µM Myr-FRCRCFa, I Na-Ca was reduced to 4.2±0.3 pA/pF at +60 mV and –1.5±0.2 pA/pF at –100 mV (P<0.02; n=7). After incubation with 20 µM Myr-FRCRCFa for 1 h, I Na-Ca at both potentials was further reduced (2.3±0.8 pA/pF at +60 mV; –0.9±0.3 pA/pF at –100 mV; P<0.008 compared with control; n=4). Under selective recording conditions for I Ca,L, there was little difference in I Ca,L density between untreated and cells incubated with Myr-FRCRCFa. A Boltzmann fit to the I Ca,L/V relation showed no significant alteration of half-maximal activation potential or slope factor of activation. I K1 was also largely unaffected by pre-incubation of cells with Myr-FRCRCFa. I K, measured as deactivating tail current following 1-s test depolarisations to a range of test potentials, was also not significantly altered by Myr-FRCRCFa. The suppression of I Na-Ca in cells incubated in Myr-FRCRCFa suggests that addition of the myristyl group to FRCRCFa peptide conveys cell permeancy to the peptide and that Myr-FRCRCFa applied externally to rabbit ventricular myocytes is moderately effective as an I Na-Ca blocker. I Ca,L, I K1 and I K were largely unaffected by Myr-FRCRCFa. N-Myristylation of such conformationally constrained hexapeptides may, therefore, provide a means of producing cell-permeant inhibitors of the cardiac Na-Ca exchanger.