The non-competitive NMDA receptor antagonist, MK-801 (dizocilpine), induces in rats a characteristic behavioural syndrome with ataxia, stereotypies and hyperlocomotion. At least part of this behavioural syndrome is thought to be related to interactions between glutamatergic and dopaminergic neurotransmission. Based on recent biochemical evidence that serotonin (5-HT) might also be involved in the effects of MK-801 several 5-HT receptor ligands were tested for effects on MK-801-induced behaviours. The 5-HT1A receptor ligands, ipsapirone and NAN-190, which are known to display antagonist-like properties in functional models of postsynaptic 5-HT1A receptor activity attenuated or blocked the hyperlocomotion and head weaving observed after administration of MK-801, whereas the 5-HT2 receptor antagonist, ritanserin, was ineffective in this respect. The dopamine receptor antagonist, haloperidol, and the alpha 1-adrenoceptor antagonist, prazosin, also attenuated behaviours induced by MK-801. In contrast to its effects on stereotypies induced by MK-801, ipsapirone potentiated rather than attenuated the stereotyped behaviour induced by the dopamine receptor agonist, apomorphine, indicating that antagonism of MK-801-induced stereotypies by ipsapirone may not be related to the dopaminergic system. The data indicate that, in addition to catecholaminergic systems, serotonergic neurotransmission is significantly involved in the mechanisms by which MK-801 alters behaviour in rats.