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1. The upstroke of the action potential was used as a signal of variable frequency in order to measure the apparent membrane capacity of cardiac Purkyn fibres.

2. The upstroke in Na-Tyrode can be regarded as a high frequency signal. Action potentials in Na-free, Sr-Tyrode are characterized by a very small rate of depolarization (5 V/sec) and conduction velocity (15 cm/sec) and were used as a low frequency signal.

3. On changing from Na- to Sr-Tyrode the apparent membrane capacity increased by a factor of 3·77 and approached the value estimated by the square-current technique.

4. Cable properties (space and time constants, internal core and membrane resistance) were found to be constant when changing from Na- to Sr-Tyrode.

5. The significance of the frequency dependent character of the membrane capacity is discussed in relation to propagation and form of the action potential.