Activity of serotonergic dorsal raphe neurons was recorded intracellularly in a brainstem slice preparation from rats before and after local microperfusion of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). The initial effect of the drug consisted of a transient hyperpolarization. Over 1-2 h after drug application there was a gradual decrease in efficacy of a hyperpolarizing current pulse to evoke a transient rectification. Also, action potentials in some cells failed to evoke a spike after-hyperpolarization. These effects are related to the neurodegenerative action of 5,7-DHT.