The role of prolactin (Prl) in the regulation of Prl binding to its specific binding sites was studied in the Langerhans islets, adrenal gland and liver of adult ovariectomized female rats. Animals were sc injected twice daily during 10 days with ovine Prl (1 mg/kg BW), sulpiride (30 mg/kg BW) and bromocriptine (3 mg/kg BW). At the end of the treatment period, the animals were killed and serum was collected for Prl assay. Total Prl binding sites were measured in the membrane fraction of tissue by desaturating the occupied membrane receptors in vitro with 4M MgCl2. Serum levels of Prl were significantly higher in sulpiride-treated animals, whereas bromocriptine administration rendered undetectable values. Prolactin and sulpiride treatment significantly reduced Prl binding to the adrenal gland and Langerhans islets, whereas it greatly increased Prl binding to the liver. On the other hand, bromocriptine increased Prl binding sites in the adrenal gland and Langerhans islets, but in the liver caused no apparent effect. The binding affinity (Ka) in each tissue remained unchanged under the different experimental conditions. In addition, the binding of Prl to pancreas islets membranes was lower in late pregnancy when compared with control rats. All of these data provide strong evidence in favor of a role for Prl in regulating the number of its own tissue binding sites.