The purposes of the present study were to determine (a) whether changes of intracellular [Ca2+] (Cai) can account for the decrease of developed tension observed in rat heart muscle when stimulation rate is increased, and (b) whether the effect of stimulation rate on Cai is altered in conditions in which the rate of repriming of the sarcoplasmic reticulum (SR) is altered, as when perfusate [Ca2+] (Cao) is increased, and in heart muscle from senescent animals. The photoprotein aequorin was used to monitor Cai in rat papillary muscles. In muscles from 6-mo-old rats, increasing the stimulation rate in the range 0.2-0.66 Hz led to parallel decreases of both the aequorin light transient and developed tension when Cao was 2 mM. When Cao was increased to 4 mM, changes in the stimulation rate had less effect on both the light transient and tension. At 8 mM Cao, changing the stimulation rate had no effect on either the light transient or developed tension. Papillary muscles from 24-mo-old rats, in which SR function is likely to be depressed, exhibited a prolonged Ca2+ transient and twitch. At a Cao of 4 or 8 mM, increasing the stimulation rate from 0.33 to 0.66 Hz still led to decreases in the size of the aequorin light transient and developed tension in these muscles. Developed tension and aequorin light responded to increases of Cao in the same way in both groups of muscles. We conclude that under the conditions of our experiments, developed tension is determined by Cai. The negative interval-strength relationship observed when Cao is in the physiological range can be accounted for by a time-dependent recycling of Ca2+ by the SR. The effects of increasing Cao and the age-related differences observed at high Cao can also be accounted for using this model.