Tethered sows continuously performed stereotypies during a substantial part of the day time. A single subcutaneous injection with naloxone significantly decreased the profound stereotypies, while explorative behaviours of pigs were not affected by naloxone. In addition a long-term effect of naloxone was noticed. It is suggested that performing the seemingly purposeless stereotypies may function as an effective strategy in order to cope with the conflict-inducing housing conditions. The present data implicate endorphins in this strategy. The involvement of endorphins in the stereotypies of the sows may be related to the calming, rewarding and dependency-inducing effects of the endorphins.