Abstract: dl-Tetrahydropalmatine (THP), a short-term monoamine depletor, has been found to be able to decrease the firing rate of the neurons in the nucleus accumbens (A). Influence of exogenous monoamines on the THP-induced inhibition of single-unit activity in A was studied by means of extracellular recording techniques in conscious rats immobilized by flaxedil. Intracerebroventricular (icv) administration of noradrenaline (NA, 10 μg/10 μl) reversed the depression of the firing in A induced by THP 30 rain after systematic injection. 5-Hydroxytryptamine (5-TH, 10 μg/ 10 μl) given icy produced a reversal of the depression of firing only by 50% 15 min after injection. While dopamine (DA, 10 μg/ 10 μl) showed no effect on the THP-induced inhibition.Our results indicate that NA and 5-TH, rather than DA, may be responsible for the THP-induced inhibition of single-unit activity in A.