The number of nucleolar organizer regions reflects nuclear and cellular activity, such as the proliferation and differentiation of cells. Prolonged administration of estrogen (E₂) induces the hyperplasia of prolactin (PRL) cells and the development of PRL-secreting pituitary tumors in rats. Dopamine agonists are known to reduce the effects of E₂ treatment. The purpose of this study was to investigate whether the changes of argyrophilic nucleolar organizer regions (AgNORs) in PRL cells are related to circulating levels of PRL or to the proliferative activity of PRL cells during the administration of stimulatory (E₂) or inhibitory (dopamine agonist) treatment in rats. E₂ increased the size and number of AgNORs per nuclear profile in PRL cells in rats. Bromocriptine and cabergoline, which are dopamine agonists, each reduced the number and size of AgNORs in PRL cells treated with E₂ for 10 weeks. In rats treated with E₂ alone or dopamine agonists followed by E₂, the number and size of AgNORs were correlated more closely with serum levels of PRL than with the proliferative activity of PRL cells. However, neither the number nor size of AgNORs in PRL cells was related with these parameters in different ages of the control. The number and size of AgNORs may be useful in evaluating the secretory activity of pituitary cells during the administration of stimulatory or inhibitory agents.