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1. The sequestration of activated intracellular Ca^++ ions in isolated Rabbit’s cardiac papillary muscle cells, primed with square waves in normal Tyrode solution, histamine-Tyrode (3μM/L) and lanthanum-Tyrode (50μM/L), was studied by contractile responses to various test pulse interval. 2. In normal Tyrode, the disappearance of activated intracellular Ca^++ ions occurred in two pattern, i.e., the initial one was the fast component in which the contractility decreased rapidly within 5 sec. of test pulse, interval and the next, slow component in which the contractility decreased rather slowly following fast component. 3. In histamine-Tyrode, the contractility increased to 110.3±6.0% as compared with that in normal Tyrode. The fast component of Ca^++ ion disappearance was prolonged up to 10 sec. and a significant increase in contractile responses to test pulse manifested during this component. The slow component, however, was similar to that in normal Tyrode. 4. In lanthanum-Tyrode, the contractility decreased to 65.2±6.7% as compared with that in normal Tyrode. The fast component of Ca^++ ion disappearance was complete within 3 sec. of test pulse intervals and the slow component similar to that in normal Tyrode followed. 5. The data appear to indicate that the initial, rapid disappearance of activated Ca^++ ions, the fast component, was the result of sequestration of intracellular Ca^++ ions influxed across cell membrane from outside, and the next, slow component represented the sequestration of Ca^++ ions freed from intracellular storage site.
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