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The control isometric contraction of rabbit papillary muscle preparations were recorded by the stimulation ’frequency of 1 Hz. and priming stimulation were applied to activate the intracellular calcium by the stimulation frequency of 5 Hz. for 5 seconds. After the resting period, the amplitudes of contraction, were compared with that of control and the ratios were plotted with respect to time in a semi-log paper. The experiments were performed in a control Tyrode solution, high calcium solution, ouabain solution and verapamil solution. The following results were obtained; 1. la a normal Tyrode solution, the fast compartment which is responsible for the fast decay of intracellular calcium within 3-5 seconds after the priming stimuli and the slow compartment, slowly decayed one after 5 seconds, could be identified. 2. Although the fast components showed no significant differences between those in a normal Tyrode solution and in a high calcium (4mM) solution, the slow components showed a rather increasing tendency during the normal initial decay period up to 20 seconds and later on a decreasing tendency. 3. In a ouabain, 2.4 -4.8×10^-7M, treated preparation the amplitude of contraction during the control period was increased up to 1.5 times compared with that in a normal Tyrode solution and the characteristics of calcium pools were qualitatively similar to those in high calcium solution. 4. In a verapamil, 1.1×10^-6M, containing solution the amplitude of control contraction were reduced to two thirds compared with that in normal Tyrode solution and the fast component were markedly influenced, reversed its slope to increase slowly up to 20 seconds and then decayed. 5. The characteristics of intracellular calcium pools la a preparation of verapamil and high calcium conditions at the same time showed rather similar patterns with those in verapamil alone treated preparation. The above results showed that the intracelluar calcium pools were composed of two components which were fast and slow decaying ones, and that ouabain and verapamil would alter the characteristics of the calcium pools to increase and decrease the intracellular calcium respectively.
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