The experimental work summarized in this paper and described in more detail in our previous publication demonstrates a very important functional role for Na (+)-Ca2+ exchange in intracellular Ca2+ homeostasis in ventricular myocytes from rat hearts. Ca2+ homeostasis in mammalian cardiac myocytes can be considered to be the result of four interactive processes:(i) Ca2+ influx through L-type Ca2+ channels,(ii) Ca2+ release from the SR and its subsequent re-uptake,(iii) intracellular Ca/+ buffering, and (iv) Ca2+ extrusion across the sarcolemma. Our results demonstrate a number of interesting features of these processes.(1) When the action potential voltage-clamp technique is used to identify the size and time-course of Ca2+ fluxes during the action potential, both the peak current and the associated influx of Ca2+ are relatively large as was previously demonstrated by Isenberg and his colleagues.(2) Nevertheless, this source of Ca2+ is unable, by itself, to produce a significant twitch, which is consistent with previous data from rat ventricle.(3) This Ca2+ influx, however, does represent the trigger for SR Ca2+ release.(4) The Na (+)-Ca2+ exchanger on the SR is able, on average, to extrude all the Ca2+ which enters through L-type Ca2+ channels, although it provides relatively little Ca2+, ie, during the course of the normal action potential there is no significant reverse Na (+)-Ca2+ exchange activity, at least under our experimental conditions. Our results also suggest that although the L-type Ca2+ current cannot by itself trigger and control contraction its amplitude, frequency, and time-course can alter the rate and the extent of Ca2+ release from the SR. Recently, detailed mathematical formulations and a direct demonstration of some of these phenomena have been published. Stern and Stern and Lakatta predicted more than three years ago that the concentration and the time-course of change in concentration of Ca2+ very near the release sites of the SR may be critical determinants of the overall release process. Within the past year Wier and his colleagues and also Lederer et al. have combined electrophysiological measurements with recordings of localized intracellular Ca2+(made using a confocal microscope) and have shown that rapid, and relatively large, but very localized changes in intracellular Ca2+ due to Ca2+ influx through L-type Ca2+ channels are responsible for triggering, and to some extent, controlling the release of Ca2+ from the SR. However, it has also been shown that this release depends importantly on the loading or priming state of the SR. Perhaps not surprisingly, the massive release of Ca2+ from the SR can, itself, alter the pattern of subsequent SR release events (cf. Ref. 46) and the time-course of Ca2+ influx through the L-type Ca2+ channels. Thus, although our relatively crude measurements have clearly demonstrated the relationship between L-type Ca2+ channel activity and Na+-Ca2+ exchanger function during a normal cardiac action potential in rat ventricle, they fall far short of any delineation of the functional roles of either of these processes in overall Ca2+ homeostasis. This additional information can, in principle, be obtained from studies in which cellular microanatomy can be visualized dynamically in conjunction with localized changes in intracellular Ca2+ as well as Ca2+ of L-type Ca2+ channels, SR release, and cell shortening.