Plots of post-tetanic changes in the amplitude of the acetylcholine-evoked influx current (the ACh current) in common snail command neurons were compared in different experimental conditions: with and without a flow of physiological saline through the chamber containing snail ganglia. Flow of physiological saline through the chamber containing the ganglia decreased the latent period and reduced the magnitude of the post-tetanic increase in the neuronal ACh current. The earlier and less marked post-tetanic potentiation of the ACh current in neurons with a flow of physiological saline through the chamber containing the ganglia are evidence for the involvement of a humoral factor in the mechanism of the post-tetanic increases in cholinosensitivity of the somatic membranes of postsynaptic neurons.