Electrophysiological techniques were used to study the changes in the terminal excitability of mesolimbic DA and non-DA neurons following the infusion of D-amphetamine (D-AMP) and haloperidol (HAL) into the nucleus accumbens (NAc) of rats. The amount of current needed to evoke antidromic spikes by electrical stimulation of the NAc was used as an index of the excitability of axon terminals of these neurons. The excitability of DA neurons was decreased by D-AMP and increased by HAL. In addition, the effect produced by D-AMP was reversed by HAL. By contrast, these drugs either induced an opposite effect or were ineffective in inducing changes on the excitability of nerve terminals of mesolimbic non-DA neurons. Infusion of the vehicle or saline produced no effect. D-AMP and HAL were still effective in modulating the excitability of mesolimbic DA nerve terminals after the destruction of NAc neurons by ibotenic acid. The results suggest that the effects seen after D-AMP and HAL are mediated primarily by DA autoreceptors. It is likely that the increase in the current needed for evoking antidromic spikes after infusion of D-AMP into the terminal region is the consequence of DA autoreceptor-mediated hyperpolarization of terminal membranes. On the other hand, HAL could exert its actions by blocking autoreceptor-mediated hyperpolarization.