The effects of ouabain and adrenaline on kinetically distinct compartments of exchangeable calcium associated with myocardial muscle cells were investigated using a Langendorff perfused guinea-pig heart preparation, a 45Ca2+ outflow exchange technique, and 51Cr-EDTA to monitor the rate of loss of freely-diffusable 45Ca2+ from the vascular and interstitial spaces. A non-linear least squares curve-fitting procedure was used to analyse the data. The minimum number of compartments of exchangeable calcium required to adequately describe each set of data for the loss of 45Ca from the heart was five. These consisted of freely-diffusible exchangeable calcium in the vascular and interstitial spaces (monitored using 51Cr-EDTA) and three kinetically distinct compartments of exchangeable calcium associated with the muscle cells. A concentration of ouabain (0.15 μM), which enhanced the force of contraction of the heart by 60% without evidence of toxicity, increased the flux and quantity of exchangeable calcium in the vascular space and in the cellular compartment with the highest fractional transfer rate. Both ouabain and adrenaline decreased the flux and fractional transfer rate of exchangeable calcium in the two cellular compartments with the lowest fractional transfer rates. The results are consistent with the conclusion that one of the actions of ouabain on myocardial muscle cells is to increase the quantity of rapidly-exchangeable calcium bound to extracellular sites on the sarcolemma and/or present in the myoplasm.