We have used the technique of quantitative in situ hybridization in order to study the changes in the levels of expression of D2 receptor mRNA in the anterior pituitary gland of female rats at different stages of the reproductive cycle. Plasma prolactin levels in the same animals were determined by radioimmunoassay. Rats in the prooestrous, oestrous, dioestrous 1, dioestrous 2 phases of the oestrous cycle and in pregnant, lactating, ovariectomised and ovariectomised animals treated with diethylstilbestrol (DES) have been examined. Our results show that expression of D2 receptor mRNA in the anterior pituitary gland varies during the oestrous cycle, with the lowest expression measured during oestrus. Expression levels increased during dioestrus 1, reaching the highest values in dioestrus 2 and declining again in prooestrus. Expression of D2 receptor mRNA was reduced during pregnancy when compared to lactating animals. In ovariectomised animals, the level of D2 receptor mRNA was similar to that observed in intact animals during oestrus. Ovariectomised animals treated with DES showed dramatically increased prolactin levels, while D2 receptor mRNA remained low. Prolactin secretion might be controlled not only by variations in the release and plasma concentrations of dopamine itself, but also by modulation of D2 receptor expression in pituitary cells. Our results suggest that the variations in D2 receptor density in the anterior pituitary cells at different physiological states are, at least to some extent, regulated at the level of gene expression.