We investigated the effect of KB-R7943, an inhibitor of the reverse mode of Na+/Ca2+exchanger, on the force of isometric contractions, the contractile force–frequency relationship and post-rest potentiation (a qualitative parameter of Ca2+levels in sarcoplasmic reticulum) in the right ventricle papillary muscles isolated from ground squirrel hearts during summer (June, n = 4) and autumn (October, n = 4) activities. In the presence of 1.8 mM Ca2+at 36°C, 1–1.5 hours-long treatment of the summer papillary muscles with KB-R7943 produced no significant effects on the contractile indices at the majority of stimulation frequencies. In the autumn papillary muscles KB-R7943 induced a 40–50% decrease in the force of contraction (negative inotropic effect) at low stimulation frequencies (0.1–0.3 Hz) without any significant effect at higher stimulation frequencies (0.4–3.0 Hz). Furthermore, in this group, KB-R7943 suppressed the post-rest potentiation of contractility by 50 ± 21% at pause durations exceeding 120 s. These observations indicate that KB-R7943 can affect Ca2+levels in sarcoplasmic reticulum and that Na+/Ca2+exchange may contribute to the physiological remodeling of intracellular Ca2+homeostasis in myocardium of hibernating animals prior their transition to a hypometabolic torpid state.