THE NMDA receptor antagonist phencyclidine (PCP) has low micromolar affinity for the 5-HT reuptake site, but it is uncertain whether PCP blocks 5-HT reuptake when given systemically to rats in behaviourally stimulating doses. We here report for the first time that systemically administered PCP (5 mg/kg, s.c.) increases extracellular 5-HT levels in the rat medial prefrontal cortex (to 322%) and dorsal hippocampus (to 233%). Increases were found also when citalopram (1 microM) was included in the perfusion medium (to 184 and 180%, respectively). Extracellular 5-HIAA concentrations increased during both conditions, and extracellular GABA decreased in the dorsal hippocampus. It is concluded that systemic PCP treatment elevates extracellular 5-HT levels, probably through mechanisms other than a blockade of 5-HT reuptake.