1. Sympathetic nerve-atrial preparations isolated from rabbits were used in the present study. Transmembrane potentials were recorded from sino-atrial (S-A) nodal pace-maker fibres.2. A reduction of [Na(+)](o) (extracellular concentration of sodium ions) decreased the maximal diastolic potential, reduced the size of the overshoot, prolonged the depolarization time, increased the rate of early repolarization and decreased the rate of late repolarization.3. A Na(+)-deficiency markedly augmented the positive chronotropic effect of stimulation of post-ganglionic sympathetic nerves at a frequency of 5/sec but slightly augmented the effect of stimulation at 1 and 20/sec.4. The marked membrane effect of sympathetic nerve stimulation at low [Na(+)](o) was an increase in the slope of slow depolarization during diastole. Corresponding to an induced acceleration of S-A nodal rate, the 90% duration of the action potential was decreased but the other action potential parameters measured were not influenced.5. A reduction of [Na(+)](o) to 51.7 mM produced in the pace-maker membrane, subthreshold oscillations the rate and the amplitude of which were increased by sympathetic nerve stimulation until conducted potentials were generated.6. It is presumed that cardiac noradrenaline elicits an increase in the slope of slow depolarization during late diastole which is due to an increase in Na(+) permeability, a decrease in K(+) permeability or a non-selective increase in ion permeability, but that it does not influence the pace-maker membrane during systole.