The effect of haloperidol, pimozide and amphetamine on acetylcholine (ACh) output from the cerebral cortex was investigated in unanaesthetized, freely moving and urethane-anaesthetized rats and guinea pigs. Haloperidol (1 mg/kg i.p.) decreased ACh output only in the anaesthetized rats and increased it only in unanaesthetized guinea pigs. Pimozide (1 mg/kg i.p.) stimulated ACh output in unanaesthetized rats and guinea pigs and anaesthetized guinea pigs, but not in anaesthetized rats. Amphetamine (1 mg/kg i.p.) in all cases, stimulated ACh output. In rats with a septal lesion, the effect of amphetamine on ACh output was suppressed but that of pimozide was still present.