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The motor activity of reserpine treated mice was recorded after drug treatments causing stimulation of dopamine or noradrenaline receptors or both. The dopamine receptor stimulating agent apomorphine elicited an activation with stereotypies whereas the noradrenaline receptor stimulating agent clonidine was inefficient. Combined treatment with apomorphine and clonidine induced marked stimulation with jumping. Biochemically, clonidine did not significantly interfere with the metabolism of apomorphine. Administration of metatyrosine, which stimulates dopamine receptors, produced an activation with stereotypies. Administration of α-methyldopa, which weakly stimulates dopamine receptors and strongly stimulates noradrenaline receptors, produced a slight activation. When metatyrosine was given in combination with clonidine, or when α-methyldopa was given in combination with apomorphine, a substantial activation was found as after treatment with apomorphine plus clonidine. The results indicate that a dopamine receptor activity is of basic importance for motor activity and that changes in noradrenaline receptor activity can markedly affect the motor activity, qualitatively and quantitatively, provided that there is a certain dopamine receptor activity. It can also be concluded that the effects of drugs on the dopamine mechanisms in the corpus striatum and on the noradrenaline mechanisms in the spinal cord are of value in predicting behavioural effects. [ABSTRACT FROM AUTHOR]