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 i_f, an inward Na- and K-current that turns on at voltages more negative than −40 mV^1–9. The ‘pacemaker’ current i_f is also present in other types of cell where its ability to produce and modulate a depolarizing process may be useful¹⁰. For example, in vertebrate photoreceptors i_f drives the depolarization that terminates the light-induced hyperpolarization¹¹. Currents similar to i_f are also found in hippocampal neurones and DRG neurones^12,13. The present report shows for the first time that the opening of single i_f-channels of low conductance (1 pS) can be resolved using a modification of the patch-clamp technique¹⁴ on isolated SA-node cells. Modulation of i_f by adrenaline is shown to be mediated by an increase in the probability of channel opening, whereas the single-channel amplitude remains unchanged.