Slowing of cardiac pacemaking induced by cholinergic input is thought to arise from the opening of potassium channels caused by muscarinic receptor stimulation. In mammalian sinoatrial node cells, however, muscarinic stimulation also inhibits the hyperpolarization-activated current (I_f), which is involved in the generation of pacemaker activity and its acceleration by catecholamines. Acetylcholine at nanomolar concentrations inhibits I_f and slows spontaneous rate, whereas 20 times higher concentrations are required to activate the acetylcholine-dependent potassium current (I_K,Ach). Thus, modulation of I_f, rather than I_K,Ach, is the mechanism underlying the muscarinic control of cardiac pacing at low (nanomolar) acetylcholine concentrations.