DSP-4 is a neurotoxin highly selective for the noradrenergic nerve terminals of the locus coeruleus projections. Data on the effect of DSP-4 treatment on amphetamine-induced hyperlocomotion are contradictory. In this study, DSP-4 (50 mg/kg) caused reduction of noradrenaline levels by 70% in the cerebral cortex and by 79% in the cerebellum. This treatment resulted in upregulation of dopamine D2 receptors in the striatum as evidenced by [3H]-raclopride binding. In an open field test, DSP-4 reduced locomotor activity. D-Amphetamine (1.5 mg/kg) caused a similar increase in locomotor activity in control and DSP-4-pretreated animals not familiar to the apparatus. However, when the rats were habituated to the test apparatus, the effect of amphetamine on horizontal activity was significantly larger in the DSP-4-pretreated animals. These data suggest that supersensitivity of D2 receptors develops after locus coeruleus denervation, but that the enhanced efficacy of amphetamine in DSP-4-treated rats is masked by neophobia.