We analyzed sarcomere shortening of isolated ventricular myocytes (vm) from young (2 mo.) and aged (24 mo.) mice. Vm were stimulated with 40 ms voltage clamp pulses from -80 to 0 mV (0.5-6 Hz; 37°C). Compared to young vm, in aged vm, the extend of shortening was significantly reduced. This difference was insensitive to the superoxide scavenger Tiron. Time to peak (TTP) and time to 90% relaxation (TTR₉₀) were similar in both groups. With increased pulse duration (160 ms; 0.5-4 Hz) aged vm shortened to the same extend as young vm. Nevertheless, this protocol induced differences in the time course of contraction. TTP as well as TTR₉₀ were significantly prolonged in aged vm. These changes were attenuated in the presence of Tiron. Expression of SERCA, PLB and CaMK II did not account for the slowed relaxation of aged vm. Thus, we analyzed Ca²⁺ induced PLB phosphorylation at Thr¹⁷ by Western blot. In response to [Na⁺]_o reduction and subsequent [Ca²⁺]_cyt increase via NCX activity, aged vm showed significant less CaMK II mediated PLB phosphorylation that could be normalized by Tiron. Conclusion: Increased pulse duration can compensate the deficien-cies in E-C coupling of aged vm on the expense of slowed contraction/relaxation. ROS dependent reduction in CaMK II me-diated PLB phosphorylation may contribute to slowed relaxation.