The effects of scopolamine pretreatment on stereotyped behavior induced by apomorphine in rats withdrawn from long-term haloperidol administration.

1. The effects of scopolamine on intensity of stereotyped behavior induced by the ED50 dose of apomorphine in controls and in animals withdrawn from long-term haloperidol treatment were studied. 2. Doses of 1.0 and 3.0 microgram/kg of scopolamine increased apomorphine-induced stereotypy in both groups. However, this increase was smaller in the haloperidol-treated than in the control animals. 3. These data support the hypothesis that long-term administration of haloperidol may produce a change not only in dopaminergic but also in central cholinergic receptor sensitivity.