One theory about the pathogenesis of endogenous depression is that decreased serotonergic (5-HT) neurotransmission is involved in producing the disorder. A key component of brain 5-HT neurotransmission is the discharge rate of 5-HT neurons in the dorsal raphe nucleus (DRN), a major aggregation of 5-HT neurons. We tested the hypothesis that the discharge rate of 5-HT neurons in the DRN was decreased in a new animal (rat) model of human endogenous depression. In this model, rats are treated neonatally with the antidepressant chlorimipramine. When adult, these animals exhibit several behavioral, REM sleep, and treatment response features of the human disorder. We found in a single unit measurements in adult, pentobarbital-anesthetized rats that, compared with 'non-depressed' control rats, the 'depressed' rats had a lower discharge rate of 5-HT neurons in the DRN. This correlation is consistent with the theory that 5-HT neurotransmission is diminished in endogenous depression.