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Diphenylhydantoin (DPH) has recently been reported to produce dopaminergic (DA) supersensitivity in animals. These results have suggested that the dyskinesias observed in humans after DPH, although rare, might be regarded as a neuroleptic-like effect. Indeed dyskinesias would be induced by an inactivation of post-synaptic DAergic receptors, operated by DPH, and therefore reminiscent of that observable in neuroleptic treatment. In order to investigate this matter, we studied the effects of i.v. DPH on the extracellular single unit activity of DAergic cells located in mid-brain areas of rats. DPH was injected alone or in combination with DA antagonists such as L-sulpiride (L-SULP) and haloperidol (HAL), or the DAergic agonist apomorphine (APO). Our results show that DPH did not affect spontaneous DAergic firing rate and also failed to modify the known action of the DA agonists and antagonists which were tested on these neurons.