Abstract: AIM　To study the effects of Phe-Met-Arg-Phe-NH₂ (FMRFa) on Na⁺/Ca²⁺ exchange and its specificity for Na⁺/Ca²⁺ exchange in rat ventricular myocytes. METHODS　Na⁺/Ca²⁺ exchange current and other currents of ion channels were measured using whole cell voltage clamp techniques. RESULTS　A dose-related inhibition of tetrapeptide FMRFa on Na⁺/Ca²⁺ exchange was observed in rat ventricular myocytes. Inward and outward I_Na⁺/Ca²⁺ were inhibited by 60.1% and 56.5%, respectively, at highest concentration (100 μmol·L^-1) and its IC₅₀ were 20 μmol·L^-1 and 34 μmol·L^-1 in inward and outward I_Na⁺/Ca²⁺, respectively. Inward and outward I_Na⁺/Ca²⁺ were inhibited 38.7% and 34.9%, respectively, at FMRFa 5 μmol·L^-1. FMRFa 5 μmol·L^-1 and 20 μmol·L^-1 did not affect L-type calcium current, sodium current, transient outward current and inward rectifier potassium current. CONCLUSION　These data indicate that FMRFa is a specific inhibitor of Na⁺/Ca²⁺ exchange in intact rat ventricular myocytes.