The effects of naloxone hydrochloride (0.01-10 mg X kg-1), naltrexone hydrochloride (0.01-10 mg X kg-1), and morphine sulphate (0.01-10 mg X kg-1) on the increased water consumption provoked by administration of chlordiazepoxide (10 mg X kg-1) were investigated in male rats which had been adapted to a 22 hr water-deprivation schedule. As previously reported, both naloxone and naltrexone dose-relatedly reduced water ingestion. Naloxone at 1 mg X kg-1 and naltrexone at 0.1 mg X kg-1 completely blocked the chlordiazepoxide-induced hyperdipsia. Since both opiate antagonists removed the chlordiazepoxide-induced effects in small doses, their effect can plausibly be attributed to opiate receptor blockade. Hence, chlordiazepoxide-induced hyperdipsia may depend upon the activation of endogenous opioid mechanisms. Morphine had little effect on drinking, unless a large dose (10 mg X kg-1) was used, when the thirst-induced and chlordiazepoxide-induced drinking were attenuated. The data provided no evidence that morphine, a mu opiate agonist, enhanced chlordiazepoxide-induced water consumption. The results are considered in relation to other relevant behavioural and biochemical findings.