The adult, male rat, trained to consume daily fluid intake in a deprivation-induced situation, was used to examine the effects of single doses of the monovalent alkali metal cations. Dosages of LiCl (0.25 - 5.0 mEq/kg, i.p.) showed a biphasic action with a threshold at 0.5 mEq/kg and a maximum dipsogenic action of 1 - mEq/kg. In contrast, single doses of NaCl (1.0 - 7.5 mEq/kg, i.p.) showed a continuing dipsogenic effect over the entire dose range. Single doses of KCl (0.25 - 5.0 mEq/kg, i.p.) or RbCl (0.5 - 5.0 mEq/kg, i.p.) had no effect on fluid consumption, while CsCl (0.5 - 2.1 mEq/kg, i.p.) caused decrements in fluid consumption with no clear dose-response relationship.