Normal pacemaking in the mammalian heart is driven by spontaneously active cells located in the sino-atrial (SA) node. The rate of firing of these cells and the modulation of this rate by catecholamines are controlled by if, an inward Na- and K-current that turns on at voltages more negative than -40 mV. The 'pacemaker' current if is also present in other types of cell where its ability to produce and modulate a depolarizing process may be useful. For example, in vertebrate photoreceptors if drives the depolarization that terminates the light-induced hyperpolarization. Currents similar to if are also found in hippocampal neurones and DRG neurones. The present report shows for the first time that the opening of single if-channels of low conductance (1 pS) can be resolved using a modification of the patch-clamp technique on isolated SA-node cells. Modulation of if by adrenaline is shown to be mediated by an increase in the probability of channel opening, whereas the single-channel amplitude remains unchanged.