&NA;[Ca2+]itransients, elicited by voltage‐clamp depolarization of single guinea pig cardiac ventricular cells, were observed through use of the fluorescent Ca2+indicator, fura‐2. Individual cells, loaded with fura‐2 either by internal perfusion or by exposure to fura‐2/AM, were studied with the use of an inverted microscope that was equipped with ultraviolet epifluorescence illumination, an intensified silicon intensifier target camera, and a photomultiplier tube. Variation of membrane voltage and exposure of cells to verapamil (a Ca2+channel blocker) and ryanodine (which was assumed to abolish selectively the release of Ca2+from the sarcoplasmic reticulum) were used to investigate the cellular processes that determine the [Ca2+]itransient. The principal results of the study are: 1) When appropriate methods are used, the properties of cytosolic fura‐2 inside guinea pig cells are similar to those of fura‐2 in solution, irrespective of the method of loading. 2) The amplitude (at 100 msec) of verapamil‐sensitive fluorescence transients elicited by pulse depolarization (range ‐30 to 80 mV) has a bell‐shaped dependence on membrane voltage (maximum at 10 mV). 3) Rapid, ryanodine‐sensitive and verapamil‐sensitive “tail transients” are elicited on repolarization from membrane potentials greater than 30 mV; their amplitude increases as the amplitude of the preceding pulse increases. 4) The amplitude of slow fluorescence transients that are insensitive to verapamil and ryanodine increases continuously with membrane potential throughout the range − 20 to 80 mV. The voltage dependence and pharmacology of the rapid transients elicited by pulse depolarization or by repolarization are consistent with their having arisen from Ca2+released from the sarcoplasmic reticulum, via Ca2+‐induced Ca2+release. The [Ca2+]itransients remaining in the presence of ryanodine and verapamil may arise from Ca2+entering via the sodium‐calcium exchanger. (Circulation Research1987;61:148‐154)