MK-801 is a ligand at phencyclidine recognition sites associated with NMDA-coupled cation channels, where it acts as a potent noncompetitive antagonist of central glutamate/aspartate (NMDA-type) receptors. Low doses (10-100 micrograms/kg IP) produced a dose-related and prolonged (greater than 1 h) enhancement of variable-interval self-stimulation responding. Higher doses (300 micrograms/kg) caused flaccid ataxia and disrupted responding. Ketamine HCl (3.0-100 mg/kg IP), a dissociative anaesthetic binding to the phencyclidine site, produced a similar response pattern, but facilitation was less prolonged and occurred over a narrower dose range. Kynurenic acid (3.0-300 mg/kg IP), a nonselective competitive antagonist of glutamate receptors, produced only depression of responding, possibly the result of kynurenate-induced blockade of central kainate and/or quisqualate receptors. The behavioural stimulant effects of MK-801 appear to be an intrinsic and essential feature of selective NMDA antagonists, and these effects of MK-801 differ qualitatively and quantitatively from the well-known facilitatory effects of dopamine-dependent stimulants.