Intracellular recordings were made from neurons in the nucleus accumbens in situ to determine how dopamine produces the selective neuromodulatory action in the accumbens observed in previous studies. Electrical stimulation of the basolateral nucleus of the amygdala was found to produce monosynaptically evoked depolarizing and hyperpolarizing postsynaptic potential sequences in a large proportion of the accumbens neurons sampled. Dopamine applied iontophoretically or released endogenously by stimulation of the ventral tegmental area produced consistent membrane depolarization and an increase in membrane conductance but not an increase in spontaneous activity of the accumbens neurons. Stimulation of the ventral tegmental area with trains of 10 pulses at 10 Hz prior to stimulation of the amygdala produced 8-58% reduction in the amplitude of the depolarizing postsynaptic potential but no change in the late hyperpolarizing postsynaptic potential. Although attenuation of the depolarizing postsynaptic potential amplitude from ventral tegmental area stimulation was often accompanied by membrane depolarization, it appeared that the two responses were not causally related. The effect of ventral tegmental area stimulation on the evoked depolarizing postsynaptic potential and the membrane potential were blocked by haloperidol indicating the involvement of dopamine. Iontophoretically applied dopamine produced responses similar to ventral tegmental area stimulation with two exceptions: (i) iontophoretically applied dopamine produced consistently stronger maximal attenuation of the depolarizing postsynaptic potential than did ventral tegmental area stimulation; and (ii) iontophoretically applied dopamine always attenuated both the depolarizing postsynaptic potential and hyperpolarizing postsynaptic potential whereas ventral tegmental area stimulation produced selective attenuation of the depolarizing postsynaptic potential only. These electrophysiological results are complementary to those from pharmacological experiments and suggest that one of several physiological functions of dopamine in the nucleus accumbens is a neuromodulatory one involving presynaptic action on non-dopaminergic terminals.