Lergotrile mesylate (2-chloro-6-methylergoline-8β-acetonitrile, methanesulphate salt) was shown to be a potent inhibitor of prolactin secretion in vivo and in vitro. The dopamine receptor blocker, pimozide, was able to reverse the inhibitory effect of lergotrile mesylate (LM) on prolactin release from rat pituitaries in vitro. Alpha-adrenergic or beta-adrenergic receptor blockers were unable to antagonize the action of LM on prolactin release. These findings indicate that ergolines such as LM inhibit prolactin release from pituitaries by activating an adenohypophyseal dopamine receptor. LM is currently undergoing clinical trial as a prolactin inhibitor and a dopamine agonist.