Intracerebroventricular (i.c.v.) injection of choline (50-150 mug), a precursor of the neurotransmitter acetylcholine, produced a time-and dose-dependent increase in plasma vasopressin levels in conscious, freely moving rats. The increase in plasma vasopressin in response to i.c.v. choline (150 mug) was inhibited by pretreatment with the nicotinic receptor antagonist, mecamylamine (50 mug; i.c.v.), but not by the muscarinic receptor antagonist, atropine (10 mug; i.c.v). The choline-induced rise in plasma vasopressin levels was greatly attenuated by hemicholinium-3 (HC-3; 20 mug; i.c.v.), a neuronal choline uptake inhibitor. Choline (50 or 150 mug; i.c.v.) produced a much greater increase in plasma vasopressin levels in osmotically stimulated or hemorrhaged rats than in normal rats. Choline (150 mug; i.c.v.) also enhanced plasma vasopressin response to graded hemorrhage; the enhancing effect of choline was also attenuated by HC-3 (20 mug; i.c.v,). Choline and acetylcholine concentrations in hypothalamic dialysates increased significantly following i.c.v. injection of choline (150 mug). It is concluded that choline increases plasma vasopressin levels by stimulating central nicotinic receptors indirectly, through the enhancement of acetylcholine synthesis and release, and augments the ability of osmotic stimulations or hemorrhage to stimulate vasopressin release. (C) 2002 Elsevier Science B.V. All rights reserved.