The present experiments were performed to investigate effects of (+/-)-2-(dipropylamino)-7-hydroxy-1,2,3,4-tetrahydronaphthalene (7-OH-DPAT) or quinpirole (LY 171555), putative dopamine (DA) D3 receptor agonists, on serum prolactin levels in male rats. Basal prolactin levels were reduced dose-dependently by SC administration of 7-OH-DPAT or quinpirole at respective doses of 10-100 micrograms/kg and 25-250 micrograms/kg. Daily treatment with estradiol, 35 micrograms/kg/day for 3 days, increased serum prolactin levels to fourfold higher levels than those of nonprimed rats. Intraperitoneal injection of alpha-methyl-p-tyrosine (alpha-MT), 300 mg/kg, also increased serum prolactin levels. 7-OH-DPAT or quinpirole at a dose of 50 micrograms/kg caused a marked reduction in serum prolactin levels in both the estradiol- and alpha-MT-induced hyperprolactinemia. The 7-OH-DPAT- and quinpirole-induced decreases in serum prolactin levels were antagonized by the administration of the DA D2 receptor antagonist, spiperone, at 0.5 mg/kg. The results indicate that 7-OH-DPAT and quinpirole decrease prolactin levels in rats by stimulation of the D2 receptor.