Biochemical and myocardial functional changes were determined in rabbits made diabetic with alloxan (100 mg/kg, intravenously, two injections 24 h apart). Alloxan-induced diabetes was characterized by a state of hypoinsulinaemia and hyperglycaemia. After 10 weeks of diabetes, significant decreases in heart and left ventricular weights as well as increased serum and heart triglycerides and cholesterol were observed in the diabetic animals (p less than 0.05). In addition, left ventricular pressure, heart rate and rate of left ventricular pressure development were all decreased in the animals. The diabetic state was also associated with a slight elevation in myocardial calcium and a significant decrease in magnesium levels (p less than 0.05). Subcellular fractionation of diabetic hearts indicated the presence of alterations in myofibrillar and sarcoplasmic reticulum marker enzymes (p less than 0.05). Among the lysosomal enzymes, measured, N-acetyl-beta-glucosaminidase activity was significantly increased in the homogenates of diabetic left ventricles (p less than 0.05). These alterations in hearts of diabetic rabbits may be responsible for some aspects of diabetic cardiomyopathy.