The role of the alpha 2-adrenergic system in regulating prolactin release has been studied in vivo in male rats. Yohimbine administration alone, at doses ranging from 0.2 to 5.0 mg/kg, resulted in a dose-related elevation of plasma prolactin levels from a basal level of 8 +/- 2 to 65 +/- 6 ng/ml at the highest dose. In the same experiment clonidine, 0.2 mg/kg, suppressed basal prolactin levels to 4 +/- 1 ng/ml and returned prolactin levels of all animals receiving 0.2-5.0 mg/kg yohimbine to basal levels. Rats were treated with increasing doses of clonidine (0.05-1.0 mg/kg) in the presence or absence of a constant dose (1.0 mg/kg) of yohimbine. Clonidine alone at doses of 0.05 and 0.2 mg/kg again significantly suppressed prolactin levels, while a dose of 1.0 mg/kg did not (failure of high dose clonidine to suppress prolactin levels suggests an additional effect of clonidine on prolactin secretion unrelated to alpha 2-adrenergic agonist action). All three doses of clonidine completely reversed yohimbine-induced prolactin release. Basal prolactin levels were also significantly reduced by the selective alpha 2-adrenergic agonist UK-14,304 at a dose of 0.2 mg/kg. Yohimbine-induced prolactin release was reversed by UK-14,304 at doses of 0.2 and 1.0 mg/kg, but not at the lowest dose studied, 0.05 mg/kg. The lower potency of UK-14,304 than clonidine in this assay is consistent with the lower potency of UK-14,304 as an alpha 2-adrenergic-agonist antihypertensive agent. Several alpha 2-antagonists in addition to yohimbine were studied.(ABSTRACT TRUNCATED AT 250 WORDS)