Abstract The influence of the estrous cycle on the density of dopamine D 2 sites in the rat pituitary was studied by quantitative autoradiography using [ 3H]spiperone. While no significant changes were found in the neural and intermediate lobes during the cycle, a lower number of D 2 receptors in the anterior lobe was observed in the proestrus when compared to the diestrus. The difference in the density of [ 3H]spiperone binding in the pituitary between male and female rats was also analysed. A significantly lower number of receptors in the three lobes of the male pituitary, as compared to those of the female, was found. Finally, no significant variation in the number of binding sites in the 3 pituitary lobes was observed between lactating and separated mothers.