The present experiments were aimed at the behavioural effects of clonidine and the possible involvement of opiate systems therein. Previous work had shown that opiate antagonists could block the effects of clonidine on blood pressure. In the open-field, clonidine treatment induced a decrease in ambulation and rearing activity. This effect was similar in normotensive Wistar and Wistar-Kyoto rats and in spontaneously hypertensive rats. In neither strain did naltrexone pretreatment influence the behavioural action of clonidine, however. These results do not lend support to the proposed interaction between opiate systems and the central effects of clonidine.