This article summarizes some of the effects of stimulating muscarinic receptors on nerve cell activity as observed by recording from single nerve cells and cholinergic synapses in the peripheral and central nervous sytems. It addresses the nature of the muscarinic receptor(s) involved and the ion channels and subcellular mechanisms responsible for the effects. The article concentrates on three effects: postsynaptic excitation, postsynaptic inhibition, and presynaptic (auto) inhibition. Postsynaptic excitation results primarily from the inhibition of potassium currents by M(1)/M(3)/M(5) receptors, consequent upon activation of phospholipase C by the G protein Gq. Postsynaptic inhibition results from M2-activation of inward rectifier potassium channels, consequent upon activation of Gi. Presynaptic inhibition results from M(2) or M(4) inhibition of voltage-gated calcium channels, consequent upon activation of Go. The segregation receptors, G proteins and ion channels, and the corelease of acetylcholine and glutamate from cholinergic fibres in the brain are also discussed.