The relaxation phase and its load dependence were studied in papillary muscles isolated from the left ventricle of rats of the following ages: 20 days, 2, 8, 18, and 24 months. The myofibrillar ATPase activity and the force-velocity relation were determined in each age group in order to characterize the kinetic properties of the contractile material. Both shortening velocity and myofibrillar ATPase activity showed a progressive reduction with maturation and aging. This observation suggested an age-dependent decrease in cross bridge formation rate. The relaxation phase was characterized by its duration and the maximum rate of tension decline in isometric conditions, and by the speed of relengthening in isotonic conditions. Relaxation became faster and of shorter duration with maturation from 20 days to 2 months and then became slower and of longer duration with further maturation and aging. The sensitivity of relaxation to changes in length or load was evaluated by measuring how much earlier tension declined in the presence of a given length change. An increase in load sensitivity of relaxation was observed during maturation from 20 days to 8 months. This increase was followed by a reduction during aging from 8 to 24 months. Such a biphasic trend of the age-related changes in load sensitivity of relaxation could result from the interplay between the progressive decrease in cross bridge formation rate and a reduction in activation decay rate. The latter was suggested by the prolongation of the relaxation phase and by the maintenance of developed tension during aging.