Neurotransmitters open transmembrane ion channels in target membranes. At the motor endplate, the open time of channels activated by acetylcholine determines the time course of the endplate current. Endplate channels are selective for cations and their conductance and open times are influenced by the nature of the permeating cation. Similar effects have been seen at anion-selective inhibitory synapses. Some 'blocking' drugs that depress postsynaptic responses to neurotransmitters produce effects not consistent with a simple model in which they block open channels. Channels activated by neurotransmitters can show subconductance states that may reflect the fundamental mode of operation of a variety of ion channels.