Focal stimulation of the dorsal regions of the nucleus accumbens of the rat, in vitro, evoked field potentials consisting of two negative waves. The first wave probably reflected electrical activation of the presynaptic fibres. This wave was resistant to excitatory amino acid antagonists and sulpiride. The second negative wave arose from a slower positive going component. These probably evince a population spike and field EPSP, respectively. Intracellular studies also indicated the occurrence of EPSP's and action potentials. The presumed field EPSP and population spike were reduced by kynurenic acid (IC50 approximately 300 microM) but not by AP5 nor sulpiride. When the Mg2+ was removed, the field EPSP was prolonged in a manner fully reversible by the addition of 30 microM D(-)AP5. These results indicate that focal stimulation within the nucleus accumbens may activate excitatory amino acid-ergic fibres that make synaptic contact with an indigenous population of neurones. The postsynaptic receptors were probably of a non-NMDA subtype. However NMDA receptor-mediated responses were evident upon removal of the Mg2+, suggesting that these receptors could also contribute to neuronal excitation under the appropriate physiological conditions.