Mechanical restitution and post-rest potentiation in isolated rat atria were studied in order to understand the intracellular Ca2+ handling during stimulation and interbeat interval. Various agents known to affect transmembrane Ca2+ inward current or Ca2+ accumulation of the sarcoplasmic reticulum were examined. The tissues were stimulated at 1 Hz and ectopic stimuli of different preceding intervals were driven by a programmable stimulator. The relationship between the force produced by the ectopic contraction and the duration of the preceding interval was plotted to construct mechanical restitution curves. Mechanical restitution curve was well fitted to two exponential processes, i.e., an early rapid phase followed by a slowly rising phase. It is suggested that the time constant in early phase (tau 1) of mechanical restitution curve is dependent on the reactivation of transmembrane Ca2+ inward currents as well as the translocation of Ca2+ within the sarcoplasmic reticulum. However, the time constant in late phase (tau 2) involves the rate of Ca2+ influx or efflux possibly via the Na+/Ca2+ exchange mechanism. The present studies suggest that in rat myocardium, verapamil enhances the buffering capacity of the sarcoplasmic reticulum inside the cell, while isoproterenol appears to short circuit the buffering barrier of the sarcoplasmic reticulum and enhance the flow of Ca2+ into the cytosol. Ryanodine, which accelerates the Ca2+ release from the sarcoplasmic reticulum, is believed to attenuate its buffering capacity. The present analytical methods to which the mechanical restitution and the post-rest potentiation are combined would represent a good model for the study of beat-to-beat intracellular Ca2+ handling in cardiac muscle.