Cytosolic free calcium concentration was determined in isolated ventricular myocytes from adult rats with the calcium-sensitive indicator, quin 2. The fluorescence signal from resting cells indicated that cytosolic free calcium concentration was 181 +/- 18 nM (mean +/- SEM, n = 18). Inhibition of the sodium-potassium pump with strophanthidin (0.1 mM) resulted in an increase of cytosolic free calcium concentration from 186 +/- 17 to 736 +/- 129 nM (n = 6). The results indicate that it is possible to measure cytosolic free calcium concentration in cardiac muscle cells that have been isolated enzymatically. Moreover, they confirm the observation that inhibition of the sodium-potassium pump increases cytosolic free calcium concentration, presumably via the sodium-calcium exchange mechanism.