In the Syrian cardiomyopathic hamster heart, abnormal cellular calcium regulation, resulting in cellular calcium overload, is believed to play a role in the pathogenesis of cardiac hypertrophy and failure. Alternatively, the primary abnormality may be coronary vasospasm, resulting in reperfusion-induced necrosis. According to the latter hypothesis, only those cells that suffer an ischemic insult would contain elevated calcium levels. To determine whether a generalized elevation in myocytic calcium exists in myopathic hamster hearts, we measured cellular and subcellular calcium concentrations by electron probe microanalysis in cryosections of 50 -day and 96-day myopathic and control hearts, rapidly frozen in vivo. Total calcium content of ventricular homogenates from each group was also measured by atomic absorption spectropho-tometry. No significant differences in subcellular calcium were found by electron probe microanalysis among 50-day and 96-day myopathics and then- age-matched controls. In 50-day myopathic and control hearts, mitochondrial calcium was 0.7±0.2 and 0.9±0.2, respectively, and A-band calcium was 3.0±0.4 and 2.6±0.4 nunol calcium/kg dry wt(±SEM). Results from 96-day animals were similar. Localized regions of elevated calcium were found only at sites of necrotic foci: in Na+-loaded cells (mitochondria: 4.7±1.3 (SEM) mmol/kg dry wt), in dying cells (mitochondria: 72±22 (SEM) mmol/kg dry wt) or as extracellular deposits (7-10 mol/kg dry wt). Total calcium content of hearts from myopathic hamsters, as determined by atomic absorption spectrophotometry, was also 13 times (50-day) and 50 times (96-day) higher than controls. These results demonstrate that there is a marked heterogeneity in cellular calcium content in myopathic hamster hearts, but the data do not support the hypothesis of a generalized cellular calcium overload.