Morphine-induced catalepsy was demonstrated in mice under appropriate conditions. Dose-response curves were obtained within specific limits of dose. Central muscarinic blockade with atropine sulfate but not peripheral blockade with atropine methyl bromide enhanced the cataleptic response. Centrally acting physostigmine and peripherally acting neostigmine also enhanced the catalepsy. The results with neostigmine suggest a peripheral site of action with these drugs. Amantadine and d,1-dopa enhanced the catalepsy. These appeared similar to the atropine in that they caused increased rigidity and hyperreflexia. Aminooxyacetic acid enhanced while subconvulsive doses of picrotoxin and bicuculline had little effect. Five-hydroxytryptophan had no effect.