Using the rat cerebellar Purkinje cell as a model neuronal system, the effects of norepinephrine (NE) and fluphenazine, an antipsychotic drug, were examined. Fluphenazine was a potent, specific antagonist of the NE-mediated inhibition of Purkinje cell discharge. NE, despite its inhibitory action on spontaneous firing of the Purkinje cell, increased its responsiveness to afferent synaptic pathways. NE may increase the brain''s sensitivity to incoming information, and antipsychotic drugs may interfere with this effect. A role for NE, and dopamine, in the genesis of psychosis was suggested on the basis of their mutual antagonism by antipsychotic drugs. The facilatory effects of NE on neuronal information-processing were used to predict a possible participation of NE in the perceptual difficulties noted in psychosis.