Contractile dysfunction is a frequently reported complication of diabetic cardiomyopathy and many ofthe defects observed in the clinical setting have also been reported in experimentally-induced diabetes.We have investigated the relationship between intracellular Ca<sup>2+</sup> concentration and cell length duringthe relaxation phase of contraction in ventricular myocytes from streptozotocin (STZ) – induced diabeticrats. Cell length and intracellular Ca<sup>2+</sup> concentration were measured simultaneously in electricallystimulated (1 Hz) myocytes loaded with fura-2 and maintained at 35-36 °C. The amplitude and time topeak shortening and Ca<sup>2+</sup> transient were similar, however, the relaxation of contraction and the Ca<sup>2+</sup> transient were significantly prolonged in myocytes from STZ-treated rats compared to controls.Myofilament Ca<sup>2+</sup> sensitivity, which was assessed by plotting cell length against fura-2 fluorescenceratio during the relaxation phase of a contraction, was significantly increased in myocytes from STZtreated(9.23 ± 0.77 mm/fura-2 fluorescence unit) compare to controls (4.84 ± 0.78 mm/fura-2 fluorescenceunit). The slower time course of relaxation of contraction and Ca<sup>2+</sup> transient may be explained bydefective sarcoplasmic reticulum Ca<sup>2+</sup> uptake and to a lesser extent mechanisms of plasma membraneCa<sup>2+</sup> efflux including Na<sup>+</sup>/Ca<sup>2+</sup> exchange and CaCa<sup>2+</sup>ATPase. In conclusion, the apparent increase inmyofilament Ca<sup>2+</sup> sensitivity may be attributed to slower cross-bridge cycling rate which in turn may berelated to the alteration of expression of different myofilament myosin isoforms.