It was found earlier that imipramine, amitriptyline and citalopram enhanced the locomotor hyperactivity induced by MK-801, a non-competitive NMDA receptor antagonist, in rats. Now, three other antidepressants: (+)-oxaprotiline, an inhibitor of the uptake of noradrenaline, (-)-oxaprotiline, an enantiomer devoid of any effect on the uptake of noradrenaline and fluoxetine, an inhibitor of the uptake of 5-hydroxytryptamine, have been examined in male Wistar rats. All those antidepressants, given in a single dose, increased the MK-801-induced locomotor hyperactivity. That increase was completely antagonized by haloperidol and partly by SCH 23390 and (+/-)-sulpiride; prazosin was inactive. Repeated administration of antidepressants produced a similar but more potent (than acute one) enhancement of the action of MK-801. Also, in that case haloperidol and SCH 23390 produced the strongest antagonistic effect; (+/-)-sulpiride and prazosin had a distinctly less potent action. Another effect of MK-801, anticonvulsant activity (electroshock-induced convulsions), was not increased by the antidepressants studied. These results indicate that antidepressants with a different pharmacological profile, increased the MK-801-induced locomotor hyperactivity, this effect being probably indirectly mediated, at least in part, by a dopamine mechanism.