The spread of electrotonic potentials during diastolic depolarization was measured in canine Purkinje fibres immersed in normal Tyrode solution. The results indicate that the space constant is gradually enhanced during the pacemaker potential reaching an optimum before the firing of the action potential. The increase in space constant is mainly due to a rise in rm but a decline in ri was also found. Epinephrine (1 microgram/ml) increased the spread of electrotonic potentials during diastolic depolarization. Assuming that the myoplasmic resistivity is not changing during the pacemaker potential the results obtained might indicate an increase in junctional conductance during this phase of cardiac cycle.