Opiate antagonists, such as naloxone, have been employed to indicate the possible involvement of endogenous opioids in a variety of behaviours including the pathogenesis of schizophrenia. This paper describes two experiments which were performed to determine the effects of naloxone on fluid intake and activity in rats. In experiment 1, the administration of 1mg/kg naloxone significantly (p<0.001) reduced water intake. 10mg/kg naloxone considerably reduced water intake, although this result was not significant. This influence was transient, since water intake was restored to control levels at the end of the 4 hours test period, and not dose related. A low dose of naloxone 0.01mg/kg produced no effect. In experiment 2, doses of 1 and 10mg/kg did not influence locomotor activity, rearing or grooming in the open field. These results suggest that naloxone may exert a primary antidipsogenic action that does not depend upon any suppression of concomitant activity. Furthermore, mechanisms controlling water intake and activity appear to be dissociated. A speculative role for the endogenous opioid system is discussed.