In the present paper, we studied in rats the effect of third ventricle administration of m-chlorophenylbiguanide hydrochloride (1-(3-chlorophenyl)biguanide (m-CPBG), a selective 5-HT(3) agonist, on water intake induced by three different physiological stimuli: water deprivation, acute salt load and hypovolemia. Central acute m-CPBG injections in the doses of 80 and 160 nmol significantly reduced water intake elicited by an acute salt load. Third ventricle injections of m-CPBG in the dose of 160 nmol significantly inhibited water intake in hypovolemic animals, whereas third ventricle injections of m-CPBG in a higher dose (320 nmol) were necessary to decrease water intake in water-deprived rats. Pretreatment with 1-methyl-N-[8-methyl-8-azabicyclo(3.2.1)-oct-3-yl]-1H-indazole-3-carboxamide (LY-278,584), a selective 5-HT(3) antagonist, abolished the inhibitory effect on water intake seen after central administration of m-CPBG in all groups studied. The central administration of m-CPBG was also able to inhibit water intake induced by pharmacological activation of central cholinergic and angiotensinergic pathways. Third ventricle injections of m-CPBG in the highest dose employed in this study (320 nmol) were unable to modify food intake in food-deprived rats. An aversion test has shown that acute third ventricle injections of m-CPBG do not induce illness-like effects that could explain the water intake inhibition here observed. Also, central administration of m-CPBG did not modify the intake of a "dessert" meal consisting of diluted condensed milk. It is concluded that central 5-HT(3) receptor activation exerts a specific inhibitory effect on water intake.