1. The muscarinic depolarizing action of ACh on cortical neurones is associated with an increase in membrane resistance (mean DeltaV/DeltaR = 3.16 mV/MOmega).2. ACh also promotes repetitive firing by slowing repolarization after spikes.3. The depolarizing effect has a mean reversal level of -86.7 mV (with mean resting potential -56 mV).4. It is concluded that as a muscarinic excitatory agent, ACh probably acts by reducing the resting K(+) conductance of cortical neurones, and also the delayed K(+) current of the action potential.5. These results are discussed in relation to the possible role of ACh in cortical function.