The present experiments were conducted to investigate the possible role of endogenous opioid peptides in the social transmission of food preferences in female mice. In the first experiment, observer animals were injected with 0.5 mg kg(-1) naltrexone, a long-lasting opioid receptor antagonist, 30 min prior to a 15-min period of social interaction with a familiar conspecific (demonstrator) fed either with a cocoa-flavoured or a plain diet. Afterwards, observers underwent a 30-min two-choice test between the cocoa and a more palatable, unfamiliar cinnamon diet. The results showed that opioid receptor blockade decreased the amount of cocoa diet consumed by the animals whose demonstrators ate cocoa diet. Experiment 2 showed that naltrexone, administered after the social interaction, strongly attenuated the expression of both a socially acquired preference for the cocoa diet and the spontaneous preference for the cinnamon diet. A third experiment was then conducted using the shorter-lasting opioid antagonist naloxone. Naloxone (0.5 mg kg(-1)) administered prior to the social interaction did not impair the acquisition of the food preference. Hence, results indicate that a blockade of opioid activity is sufficient to impair the expression of a socially acquired food preference but not its acquisition.