To determine whether the variation in the magnitude of work load sustained by the left and right ventricles during adulthood and senescence affects the load-dependent aspect of relaxation, posterior papillary muscles from the left and right ventricles of rats at 4, 10, and 20 months of age were studied under variably loaded conditions in vitro. Because of differences between the life spans of Fischer and Sprague-Dawicy rats, the functional characteristics of relaxation were investigated to evaluate the possibility of a differential age-associated response in these two strains of animals. The kinetic performance of the diastolic phase of myocardial contraction was measured by assessing the relative time during which load bearing occurred in a series of afterloaded isotonic twitches. This measurement was expressed as the ratio of the duration of afteiioaded isotonic shortening and relengthening to the time required for isometric force to decline to the same level daring isometric relaxation. A ratio of less than unity identified a load-dependent state whereas a value greater than one reflected a load-independent condition. Results snowed that the right myocardium was completely load independent whereas the left myocardium was fully load dependent at all physiological afterloads. Aging reduced the load independence of the right ventricle and the load dependence of the left ventricle in Fischer rats. In contrast, no aging effect on the properties of afteiioaded isotonic relaxation was seen in Sprague-Dawiey rats. In conclusion, distinct differences exist in the mechanical dynamics of inactivation between the left and right ventricular myocardium. Aging reduced these variations in Fischer rats but had no apparent influence in Sprague-Dawiey animals up to 20 months after birth.