Intracerebroventricular (i.c.v.) administration of choline, a precursor of acetylcholine (ACh) increased plasma prolactin levels in a time and dose-dependent manner in conscious rats. Pretreatment of rats with the cholinergic muscarinic antagonist, atropine (10 microg, i.c.v.), blocked the increase in plasma prolactin level. The increase was not influenced by pretreatment with the cholinergic nicotinic antagonist, mecamylamine (50 microg, i.c.v.). Pretreatment with hemicholinium-3 (HC-3; 20 microg, i.c.v.), a high affinity choline uptake inhibitor, attenuated the choline-induced increase of plasma prolactin levels. These results show that choline increases plasma prolactin levels by activating muscarinic receptors via presynaptic mechanisms.