The effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine ((+)-MK-801) on the firing pattern of midbrain dopamine neurons were studied with single cell recording techniques in male albino rats anaesthetized with chloral hydrate. The extracellularly recorded electrical activity of single, identified dopamine neurons was studied with respect to firing rate, burst firing and regularity of firing. MK-801 (0.01-1.0 mg/kg IV) induced different effects in different subgroups of midbrain dopamine neurons. In the substantia nigra, firing rate was increased while the pattern was regularized and burst firing slightly increased. In the ventral tegmental area, firing rate and regularity of firing was also increased while effects on burst firing were bidirectional. Histological inspections revealed that neurons which responded with an increase in burst firing were mainly located in the nucleus paranigralis subdivision of the ventral tegmental area, while cells responding with a decrease were predominantly found in the nucleus parabrachialis pigmentosus subdivision. The effects of MK-801 were similar to previously described effects of phencyclidine, another non-competitive NMDA antagonist. The present effects of MK-801 might shed some light on the mechanisms involved in psychotic symptoms induced by phencyclidine and other non-competitive NMDA antagonists.