Both naloxone (3 and 10 mg X kg-1) and naltrexone (1-10 mg X kg-1) abolished the preference for a highly palatable 0.05% sodium saccharin solution in rats that had been adapted to a 22 hr water-deprivation schedule. The effect occurred as a result of a selective decrease in the consumption of the saccharin solution, since the intake of water, which was concurrently available in the two-fluid choice test, remained unaffected. When a less preferred saccharin solution was used (0.01%), naltrexone exerted a similar suppressant effect on the sodium preference, whilst naloxone failed to produce significant effects on the intake of saccharin solution or water. The data for the opiate agonists were interpreted in terms of a drug-induced blockade of the natural reward of highly palatable fluids in thirsty rats. In the same choice test, morphine and a stabilised enkephalin analogue, with a selective agonist action at mu-opiate receptors (RX783030), failed to influence the preference for the palatable saccharin solutions. In water-deprived animals, at least, exogenous opiate agonists, active at mu-receptors, did not appear to influence the reward of the palatable solutions.