1. Single cells were isolated from rabbit sino-atrial (SA) node by enzymatic dissociation. Spontaneous action potentials and membrane currents were recorded using the whole-cell patch clamp technique to study the relationship between electrical activity and the size of the cells. 2. The size of SA node cells was estimated by measuring the cell capacitance. The cell capacitance of SA node cells ranged from 21.8 to 61.5 pF with a mean +/- S.E.M. of 38.2 +/- 1.3 pF (n = 61). 3. The action potential amplitude, maximum diastolic potential, take-off potential and action potential upstroke velocity were greater in larger cells. The rate of diastolic depolarization was greater and the intrinsic spontaneous activity was faster in larger cells. 4. The density of hyperpolarization-activated current (i(f)) was greater in larger cells, whereas the density of L-type calcium current was not correlated with the size of SA node cells. 5. TTX-sensitive sodium current (iNa) was absent in small cells with a capacitance of less than approximately 25 pF, and the density of iNa was greater in larger cells. 6. The greater density of iNa in larger cells may explain the higher upstroke velocity of the action potential in large cells, and the greater density of i(f) and iNa could be responsible for the faster intrinsic spontaneous activity of large cells. These results suggest that the SA node consists of electrophysiologically heterogeneous pacemaker cells with different electrical membrane properties.