Current has been passed through the cell membrane of muscle fibres of the isolated rabbit right ventricle with the aid of intracellular double-barrelled microelectrodes. Two types of muscle fibres were distinguished which are called P and V fibres. The relation between the intensity of a hyperpolarising current applied during the rising phase and the maximum amplitude of the action potential was different in these fibres. For P fibres the relation was essentially linear over most of the range of currents used. For V fibres the change in maximum action potential amplitude was either negligible or did not appear until a certain value of hyperpolarising current was reached. This behaviour of V fibres can be understood if a drop in polarisation resistance occurs during the rising phase and is of such short duration that the polarisation resistance has returned to its resting value before the crest of the action potential is reached. P fibres have an estimated mean resting polarisation resistance of (106 ± 13) K ohms, and a rheobase current strength of (0.08 ± 0.02) µa. In V fibres the resting polarisation resistance was (47 ± 29) K ohms and the rheobase current strength (0.47 ± 0.28) µa.

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