Caudate nucleus (CN) plays a role in neural-immune interactions and dopamine is an important neurotransmitter in it. Hence, the effect of injection of dopamine and spiperone, a dopamine antagonist into the nucleus and lesion of caudate nucleus on humoral immunity was evaluated in albino rats. Dopamine injection into caudate nucleus at a dose of 4 microg/rat/day for 6 days was found to significantly attenuate antibody response following primary immunization of rats with sheep red blood cells (SRBC), decreased the spleen and body weight ratio and total globulin levels. Bilateral lesion of caudate nucleus also had similar effects whereas injection of spiperone (4 microg/rat/day for 6 days) into caudate nucleus enhanced the primary antibody response. The observed changes following dopamine or dopamine antagonist into caudate nucleus indicate that dopaminergic pathway of caudate nucleus negatively modulates humoral immunity in rats.