It has been suggested that the sarcoplasmic reticulum (SR) is involved in the response of heart muscle to changes of perfusate [Ca2+], [H+] and stimulation rate. Two series of experiments were undertaken to investigate these suggestions. In the first, intracellular [Ca2+] (Cai) and developed tension were measured in papillary muscles from 6 and 24 month old rat hearts, during changes of perfusate [Ca2+] and stimulation rate. Heart muscle from the old animals, in which SR function is thought to be depressed, responded differently to changes of stimulation rate when perfusate [Ca2+] was high. This age-related difference was compatible with a model in which developed tension depended on Ca2+ cycling by the SR. In the second series of experiments, Cai and developed tension were measured in papillary muscles during exposure to different types of acidosis. Inhibitors of the SR were used to examine the role of the SR in the response of the muscle to acidosis. This study showed that there is an early transient recovery of tension during acidosis which is due to increased Ca2+ release from the SR. Oscillations of Cai which are generated by the SR, and which may be important in the genesis of some types of arrhythmias, could also be produced by acidosis. Preliminary analysis of data from 6 and 24 month old rats suggests that this early recovery of tension observed during acidosis may be less than in 24 month old rats.