Stimulation of the anterior cerebellar vermis (ACV) has been shown to be of therapeutic value in several patients with chronic intractable psychiatric disorders, although the mechanism of action of ACV stimulation remains obscure. The present study sought to clarify how cerebellar stimulation might function by investigating the behavioral and biochemical effects of ACV stimulation in rats. Stimulation was found to increase the amplitude of the acoustic startle response and to produce a borderline enhancement of the potentiated startle effect, results that were interpreted as evidence that ACV stimulation enhances responsiveness to significant environmental cues. A concurrent increase in dopamine turnover and a decrease in serotonin release in the nucleus accumbens suggest possible mechanisms of action of the stimulation. It is proposed that cerebellar stimulation may exert a positive therapeutic effect only in Type II schizophrenia (negative symptomatology), a category of cases possibly associated with an underactive mesolimbic dopamine pathway and, hence, not responsive to neuroleptic treatment.