The effect of humoral modulators on the morphine-induced increase in locomotor activity of mice was studied. The subcutaneous administration of 10 mg/kg of morphine-HC1 produced a marked increase in locomotor activity in mice. The morphine-induced hyperactivity was potentiated by scopolamine and attenuated by physostigmine. In contrast, both methscopolamine and neostigmine, which do not penetrate the blood-brain barrier, had no effect on the hyperactivity produced by morphine. Pretreatment of mice with alpha-methyltyrosine (20 mg/kg i.p., one hour), an inhibitor of tyrosine hydroxylase, significantly decreased the activity-increasing effects of morphine. On the other hand, pretreatment with p-chlorophenylalamine (3 X 320 mg/kg i.p., 24 hr), a serotonin depletor, caused no significant change in the hyperactivity. The study suggests that the activity-increasing effects of morphine are mediated by the release of catecholamines from adrenergic neurons in the brain. And the results are consistent with the hypothesis that morphine acts by retarding the release of acetylcholine at some central cholinergic synapses. It is also suggested from collected evidence that the activity-increasing effects of morphine in mice are mediated by mechanisms different from those which mediate the activity-increasing effects of morphine in rats.