The Ca2+ release induced by rapid cooling (RC) and caffeine in ferret ventricular muscles was investigated. For this purpose, the author measured the intracellular Ca2+ concentration ([Ca2+]i) using aequorin. Rapid lowering of the temperature of the bathing solution from 30 degrees C to low temperatures after the cessation of electrical stimulation transiently increased [Ca2+]i. The peak of [Ca2+]i was altered depending upon the temperature in RC. [Ca2+]i reached 1.59 microM when the temperature was lowered from 30 to 4 degrees C. Caffeine (15 mM) applied during cooling after the decline of RC-induced intracellular Ca2+ signal caused an increase in [Ca2+]i. The author assumed that the total Ca2+ content in the SR was the sum of the peaks of [Ca2+]i increased by RC (Ca(RC)) and caffeine application (Ca(caf)). The fractional Ca2+ release induced by RC [Ca(RC)/"assumed" total released Ca2+ from the SR(%)] was 63.7% at 4 degrees C. The peak of Ca2+ increased by RC was proportional to the "assumed" total released Ca2+ from the SR. No significant correlation was observed between RC-induced [Ca2+]i change and the [Ca2+]i before RC. However, in Na+-deficient solutions (Li+ or TMA+ replacement), the fractional Ca2+ release induced by RC was decreased; this inhibition shows a dependence upon extracellular Na+ concentration. In contrast, the fractional Ca2+ release by caffeine application after RC was enhanced. These results suggest that: 1) RC releases a fraction of the Ca2+ accumulated in the SR and 2) the mechanism of RC-induced Ca2+ release in mammalian cardiac muscle is different from that of RC-induced Ca2+ release in frog skeletal muscles, which requires an increase in [Ca2+]i before RC. Extracellular Na+ might modify RC-induced Ca2+ release through a change in [Na+]i.