The effects of verapamil on cardiac myofibrillar adenosinetriphosphatase (ATPase) activity, myosin ATPase, and myosin isoenzyme profile as well as sarcoplasmic reticular Ca2+ uptake and ATPase activities were examined in Sprague-Dawley rats made diabetic with a single injection of streptozotocin (65 mg/kg). Myofibrillar ATPase activity and myosin Ca2+ ATPase activity as well as Ca2+ uptake and Ca2+-stimulated ATPase activities of the sarcoplasmic reticulum were significantly decreased in diabetic hearts in comparison to the control values. The myosin isoenzyme component V3 was prominent in diabetic hearts, whereas V1 isoenzyme was the major myosin component in control hearts. Chronic treatment of diabetic rats with verapamil (8 mg/kg daily for 4-8 wk) resulted in an improvement of the altered myofibrillar ATPase activity, myosin ATPase, myosin isoenzyme distribution, and sarcoplasmic reticular Ca2+-pump activities in ventricular tissue. The ability of verapamil to normalize the observed defects in the subcellular organelles in diabetic cardiomyopathy may be related to its effects in controlling the entry of Ca2+ into the cardiac cell.