In the present study, we investigated the effect of intracerebroventricular (i.c.v.) administration of cytidine-5'-diphosphate (CDP) choline on plasma adrenocorticotropin (ACTH), serum growth hormone (GH), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and luteinizing hormone (LH) levels in conscious rats. The involvement of cholinergic mechanisms in these effects was also determined. In basal conditions, CDP-choline (0.5, 1.0 and 2.0 tmol, i.c.v.) increased plasma ACTH levels dose- and time-dependently, but it did not affect the TSH, GH, FSH and LH levels. In stimulated conditions, i.c.v. administration of CDP-choline (1 mumol, i.c.v.) produced an increase in clonidine-stimulated GH, thyrotyropin-releasing hormone (TRH)-stimulated TSH, LH-releasing hormone (LHRH)-stimulated LH, but not FSH levels. Injection of equimolar dose of choline (1 mumol, i.c.v.) produced similar effects on hormone levels, but cytidine (1 mumol, i.c.v.) failed to alter plasma levels of these hormones. Pretreatment with hemicholinium-3, a neuronal high affinity choline uptake inhibitor, (20 mug, i.c.v.) completely blocked the observed hormone responses to CDP-choline. The increase in plasma ACTH levels induced by CDP-choline (1 mumol, i.c.v.) was abolished by pretreatment with mecamylamine, a nicotinic receptor antagonist, (50 mug, i.c.v.) but not atropine, a muscarinic receptor antagonist, (10 mug, i.c.v.). The increase in stimulated levels of serum TSH by CDPcholine (1 mumol, i.c.v.) was blocked by atropine but not by mecamylamine pretreatment. However, CDP-choline induced increases in serum GH and LH levels were greatly attenuated by both atropine and mecamylamine pretreatments. The results show that CDP-choline can increase plasma ACTH and produce additional increases in serum levels of TSH, GH and LH stimulated by TRH, clonidine and LHRH, respectively. The activation of central cholinergic system, mainly through the presynaptic mechanisms, was involved in these effects. Central nicotinic receptors solely mediated the increase in plasma ACTH levels while the activation of central muscarinic receptors was involved in the increase in TSH levels. Both muscarinic and nicotinic receptor activations, separately, mediated the increases in serum GH and LH levels after CDP-choline.