In two experiments, phenobarbitone sodium (30 mg/kg) reliably enhanced water consumption and extended the duration of drinking in 24h water-deprived male rats. The opiate receptor antagonists naloxone (0.1-10 mg/kg) and naltrexone (0.1-10 mg/kg) both decreased water intake and reduced the duration of drinking. When the barbiturate was given in conjunction with either naloxone or naltrexone, phenobarbitone and the opiate antagonist exerted opposite effects on the two measures of drinking. While it was true that both opiate antagonists reduced water intake and drinking duration in barbiturate-treated animals, the barbiturate-induced enhancement of drinking was in no way modified by concurrent opiate antagonist treatment. Hence, the effects of phenobarbitone and of the two opiate antagonists upon the drinking measures appeared to be quite independent. There was no evidence, therefore, that the effects of phenobarbitone upon drinking were related to endogenous opioid mechanisms. The possible contrast between benzodiazepine- and barbiturate-induced hyperdipsia is briefly considered in the light of these results.