The influence of alpha receptor blockers on the facilitation of the chemoreflex by clonidine was examined in anesthetized dogs. Intravertebral arterial administration of clonidine reduced blood pressure, facilitated chemoreflex-induced vasoconstriction and inhibited carotid occlusion-induced vasoconstriction. Intravertebral arterial administration of phentolamine (0.01-0.03 mg/kg), yohimbine (0.01-0.02 mg/kg), or piperoxan (0.05 mg/kg) prevented the inhibition of carotid occlusion-induced responses by clonidine, and the latter two alpha blockers reduced the hypotensive effect. Facilitation of the chemoreflex by clonidine was decreased by yohimbine and piperoxan, but not affected by phentolamine. Intravertebral infusion of propranolol (0.1 mg/kg) had no influence on clonidine's effect on reflex responses, but appeared to accentuate its hypotensive effect. The results support a role for alpha receptor stimulation in clonidine-induced hypotension and inhibition of the carotid occlusion reflex and suggest participation of alpha receptors in the potentiation by clonidine of chemoreflex-induced vasoconstriction.