Cardiac dysfunction is a frequently reported complication of clinical and experimental diabetes mellitus. Streptozotocin (STZ) – induced diabetes in rat is associated with a variety of cardiac defects including disturbances to heart rhythm and prolonged time-course of cardiac muscle contraction and/or relaxation. The effects of carbenoxolone (CBX), a selective gap junction inhibitor, on heart rhythm and contractility in STZ-induced diabetic rat have been investigated. Heart rate was significantly (P < 0.05) reduced in Langendorff perfused spontaneously beating diabetic rat heart (171±12 BPM) compared to age-matched controls (229± 9 BPM) and further reduced by 10⁻⁵ M CBX in diabetic (20%) and in control (17%) hearts. Action potential durations (APDs), recorded on the epicardial surface of the left ventricle, were prolonged in paced (6 Hz) diabetic compared to control hearts. Perfusion of hearts with CBX caused further prolongation of APDs and to a greater extent in control compared to diabetic heart. Percentage prolongation at 70% from the peak of the action potential amplitude after CBX was 18% in diabetic compared to 48% in control heart. CBX had no significant effect on resting cell length or amplitude of ventricular myocyte shortening in diabetic or control rats. However, resting fura-2 ratio (indicator for intracellular Ca²⁺ concentration) and amplitude of the Ca²⁺ transient were significantly (P < 0.05) reduced by CBX in diabetic rats but not in controls. In conclusion the larger effects of CBX on APD in control ventricle and the normalizing effects of CBX on intracellular Ca²⁺ in ventricular myocytes from diabetic rat suggest that there may be alterations in gap junction electrophysiology in STZ-induced diabetic rat heart.