Severe Mg2+ deficiency provokes pro-oxidative and pro-inflammatory changes, and also has been shown to be pro-apoptotic in thymus and certain cell cultures. In this study we examined the extent that chronic severe dietary Mg2+ deficiency induces apoptosis in the heart. Sprague-Dawley rats were fed during three weeks with normal (25 mM, Mg-control) or magnesium deficient (2.25 mM, Mg-deficient) diets, after which, hearts were harvested and frozen. DNA fragmentation was examined in heart tissue sections, and while < 1% of nuclei were positive for apoptosis in Mg-control rat tissue, over 32% of nuclei gave positive for Klenow fragments in hearts from Mg-deficient rats. Caspase 3 activity measurements in heart homogenates showed a 3.9-fold increase in enzyme activity in Mg-deficient rat hearts compared to Mg-controls (p < 0.002); and furthermore, western blot analysis of cleaved PARP (caspase 3 substrate), showed a 4.6-fold increase of cleaved PARP in Mg-deficient rat hearts (p < 0.002). In summary, our data indicate that chronic Mg2+ deficiency induces apoptosis of myocardium in vivo.