Beta-Phenylethylamine on injection into mice produced both an early and late phase of increased locomotor activity. The early phase produced by 50 mg phenylethylamine/kg was inhibited by pretreatment with phenoxybenzamine, phentolamine, alpha-methyl-tyrosine, diethyldithiocarbamate, pimozide and haloperidol, but not by propranolol, protriptyline and reserpine. The late phase produced by 100 mg phenylethylamine/kg was blocked only by pimozide and haloperidol, both of these in doses at least ten times higher than those required to block the early phase. From these results it was concluded that the increased early phase of locomotor activity was due to the release of dopamine and noradrenaline synthetic pathway, but not intact reserpine sensitive amine stores. In contrast the late phase of locomotor activity was independent of catecholamine stores and appeared to be produced by a direct action by either phenylethylamine or metabolites on dopamine receptors.