The inhibitory effects of clonidine on the heart seem to be due to stimulation of alpha 2 adrenoceptors. alpha-Adrenoceptor blocking agents with selective affinity for alpha 2-adrenoceptors antogonize these effects. Since the alpha-adrenoceptor blocking drugs which, at the usual dosage level, antagonize the hypotensive action of clonidine are unknown in man, caution should be exercised for such an association clonidine-alpha-blocker.