We have examined the distribution of Ca2+ in voltage-clamped cardiac myocytes under resting conditions and during the Ca2+ transient. We find that the resting Ca2+ level in a quiescent rat myocyte bathed in 1 mM extracellular Ca is relatively low (between 60 and 100 nM) and uniform. At the peak of the Ca2+ transient, Ca2+ can rise to a level as high as 600 nM to 1.0 microM. Furthermore, the magnitude of the Ca2+ transient is dependent on the size of the membrane depolarization. There is good agreement between measurements made using video imaging and those made using a photomultiplier tube for the value of intracellular Ca2+ at the peak of the Ca2+ transient and for the subsequent slow changes in intracellular Ca2+. On repolarization, intracellular Ca2+ falls with a half-time of approximately 100 ms. The uniform distribution of Ca2+ reported in the Ca2+ images of myocytes at rest and at the peak of the Ca2+ transient under normal conditions is in contrast to what is observed during "Ca2+ overload" when subcellular regions of elevated Ca2+ are observed to propagate along the cell. Thus, the measurement of [Ca2+]i in cardiac myocytes with fura-2 has already yielded important new information that was not available using other techniques to measure [Ca2+]i in cardiac ventricular muscle.