Serotonin (5-hydroxytryptamine) inhibits snail neurons through two different mechanisms. Whereas on some cells it increases selectively the membrane permeability to chloride ions thus giving rise to a net influx of this ion, on other neurons it increases the permeability to potassium ions causing a net potassium efflux. The serotonin receptors involved in these two inhibitions are different; they also differ from the receptors involved in the excitatory action of serotonin previously described in snail neurons.