Trans sarcolemmal Ca movements in rabbit and rat ventricular muscle were compared using extracellular double-barreled Ca-selective microelectrodes. In rabbit ventricle, steady-state twitches were associated with transient extracellular Ca (Cao) depletions, indicative of Ca uptake during the twitch. In contrast, steady-state twitches in rat ventricle were associated with net cellular Ca extrusion. Rest periods in rabbit ventricle lead to a net loss of cell Ca and resumption of stimulation induces a net uptake of Ca by the cells. Conversely, in rat ventricle rest periods lead to cellular Ca gain and resumption of stimulation induces a net Ca loss from the cells. Thus stimulation is associated with net Ca gain in rabbit ventricle and net Ca loss in rat ventricle. These observations provide an explanation for some of the functional differences between rat and rabbit ventricle (e.g., negative force-frequency staircase and rest potentiation in rat vs. positive staircase and rest decay in rabbit). Resting intracellular Na activity (alpha iNa) was 12.7 +/- 0.6 mM in rat and 7.2 +/- 0.5 mM in rabbit ventricle. This alpha iNa in rat ventricle is sufficiently high that Ca entry via Na-Ca exchange is thermodynamically favored at the resting membrane potential. This may explain why rest potentiation is observed in rat ventricle. In contrast, the lower alpha iNa in rabbit ventricle would favor Ca extrusion via Na-Ca exchange at rest (and consequent rest decay). In rat ventricle, the increase of intracellular [Ca] ([Ca]i) associated with contraction, coupled with the short action potential duration, strongly favor Ca extrusion via Na-Ca exchange and explain the observed Cao accumulation observed during twitches in rat. The high plateau of the rabbit ventricular action potential tends to prevent Ca extrusion via Na-Ca exchange during the contraction and explains the Cao depletions observed in rabbit. It is concluded that the higher alpha iNa and shorter action potential duration in rat vs. rabbit ventricle can explain many of the functional differences observed in these tissues.