1. The aim of this work is to analyse the role that the cholinergic system could play in the production of the behaviors evoked by apomorphine and amphetamine in adult cats. These two drugs were injected s.c. in separate sessions, before and after a s.c. administration of scopolamine which blocked the muscarinic receptors. The pre and post-scopolamine results of the behaviors produced by the two catecholaminergic drugs were compared using the non-parametric Wilcoxon signed rank test. 2. In a previous step a dose-response study of the behavioral effects of scopolamine, in doses of 0.05, 0.1, 0.4 and 0.8 mg/kg was carried out in ten cats. The Kruskal-Wallis and the non-parametric multiple comparison tests were employed. A dose-dependent decrease in motility (locomotion) and a dose-dependent increase in inappetence and pupillary dilation were found. 3. In thirteen cats which were injected with 2 mg/kg of apomorphine and 2.5 mg/kg of amphetamine the findings were: 1--apomorphine after scopolamine produced a decrease in the hypermotility, compared with the results observed with the former drug previous to scopolamine; 2--with amphetamine an increase in immobility and a decrease in indifference were observed. 4. The authors conclude that the decrease in motility recorded with apomorphine and amphetamine after scopolamine, could be attributed to the proper effect of scopolamine. No explanation could be found for the decrease in indifference found by injecting amphetamine after scopolamine. 5. Considering the antagonistic effect between the dopaminergic and the cholinergic systems and that the latter one has an arousal effect, we postulate that the behavioral indifference produced by apomorphine and amphetamine could be the result of a kind of blockade of the cholinergic system when the catecholaminergic system is activated through the administration of the two cited drugs.