30-day-old polymyopathic hamsters (strain BIO 82.62) were orally treated with either magnesium pyridoxal 5-phosphate glutamate 100 mg/kg, equimolar amounts of magnesium alone (MgCl2 55 mg/kg), or water b.i.d. for 30 days, or sacrificed before treatment. 60-day-old healthy hamsters (strain CLAC) served as controls. Magnesium pyridoxal 5-phosphate glutamate lowered the myocardial calcium content (42.8 +/- 13.1 mmol/kg dry weight) as compared to MgCl2 (70.3 +/- 11.9 mmol/kg dry weight) and water (72.7 +/- 13.6 mmol/kg dry weight). The serum and tissue lipid pattern did not differ between BIO 82.62 and normal hamsters except high serum cholesterol and triglyceride levels in 30-day-old BIO 82.62 hamsters. Myocardial necroses were influenced by neither magnesium pyridoxal 5-phosphate glutamate nor MgCl2. Cytochemical investigation of the ultrastructural calcium localization in the aorta revealed less calcium precipitates with magenesium pyridoxal 5-phosphate glutamate in media myocytes. It is concluded that magnesium pyridoxal 5-phosphate glutamate exerts a calcium antagonistic effect in the cardiomyopathy of the Syrian hamster which is related neither to the magnesium content of magnesium pyridoxal 5-phosphate glutamate nor to its lipid lowering activity.