The M-current (IM), a species of time- and voltage-dependent K+-current previously identified in amphibian sympathetic neurones, has been detected in voltage-clamped rat sympathetic neurone somata. IM had an activation threshold of about -70 mV and, since it did not show time-dependent inactivation, contributed an increasing outward component to the steady membrane current between -70 and -20 mV. Selective inhibition of IM by muscarine or by angiotensin-II probably accounts for the membrane depolarization produced by these two agonists on rat ganglia.