The effects of caffeine on tension, membrane potential, membrane currents, and intracellular [Ca2+], measured as the light emitted by the Ca2+-activated photoprotein aequorin, were studied in canine cardiac Purkinje fibers. An initial, transient, positive inotropic effect of caffeine was accompanied by a transient increase in the second component of the aequorin signal (L2) but not the first (L1). In the steady state, 4 or 10 mM caffeine always decreased twitch tension and greatly reduced both L1 and L2. At a concentration of 2 mM, caffeine usually reduced but occasionally increased the steady state twitch tension. However, 2 mM caffeine always reduced both L1 and L2. Caffeine eliminated the diastolic oscillations of intracellular [Ca2+] induced by high extracellular [Ca2+]. In voltage-clamp experiments, 10 mM caffeine reduced the transient outward current and the peak tension elicited by step depolarization from a holding potential of -45 mV. In the presence of 20 mM Cs+, 10 mM caffeine reduced slow inward current. However, the time course of this reduction was far slower than that in tension and light observed in separate experiments. The simplest explanation of the results is that caffeine inhibits the sequestration of Ca2+ by the sarcoplasmic reticulum. The results also suggest that in Purkinje fibers caffeine increases the sensitivity of the myofilaments to Ca2+.