Cholinergic excitation of vertebrate neurones is frequently mediated through the action of acetylcholine on muscarinic (atropine-sensitive) receptors. This type of excitation differs substantially from the better known nicotinic excitation. One difference is that, instead of an increased membrane conductance, a decreased conductance (to K⁺ ions) frequently accompanies muscarinic depolarisation. This has been detected in sympathetic^1–4, cortical⁵ and hippocampal⁶ neurones. Using voltage-clamped frog sympathetic neurones we have now identified a distinctive voltage-sensitive K⁺ -current, separate from the delayed rectifier current, as the prime target for muscarinic agonists. We have termed this current the M-current, I_M.