Several lines of evidence have indicated that many nuclei in the brain including preoptic nucleus, AV3V, subfornical organ, septal area and lateral hypothalamus are the targets of efferents from chemo-sensitive and pressure-sensitive systems. These areas may concern with regulation of fluid homeostasis. In the present study, intracerebroventricular injections were carried out in all experiments. After a 24 h deprivation of water, the volume of consumed water was measured for 1 h. Administration of pilocarpine (0.5-1 ?g/rat), prazocin (2 ?g/rat), histamine (40-80 ?g/rat) and naloxone (0.5-1 ?g/rat) increased, while scopolamine (5-10 ?g/rat), phenylepherine (30 ?g/rat), morphine (2.5 ?g/rat), pyrilamine (25-50 ?g/rat) and ranitidine (10-20 ?g/rat) decreased water intake in isolated rats. The activation of muscarinic cholinoceptors by pilocarpine attenuated the inhibitory effect induced by phenylepherine. Blockade of muscarinic cholinoceptors did not change the phenylepherine-induced response. Pretreatment of rats with prazocin decreased the pilocarpine-induced response. Pharmacological blockade of muscarinic cholinoceptors by scopolamine decreased the prazocin –induced effect on water intake. Blockade of histamine H1 and H2 receptors attenuated the histamine-induced response. Furthermore, pyrilamine, but not ranitidine, increased the inhibitory effect induced by morphine. Pharmacological blockade of H1 and H2 receptors decreased the naloxone-induced effect on water intake. It is concluded that the histaminergic system may have a close interaction with morphine and naloxone regarding drinking behavior. Also, muscarinic cholinoceptors and adrenoceptors may have an interactive effect in this respect.