Sensitization to the cardio-accelerator action of adrenaline and noradrenaline by five antihistamines was examined on the acutely denervated heart of the cat. Antazoline (Antistin), chlorcyclizine and promethazine (Phenergan) increased the cardio-accelerator responses to both amines equally. Mepyramine (Anthisan) increased noradrenaline more than adrenaline action. Diphenhydramine (Benadryl) resembled cocaine in potentiating the responses to noradrenaline but not to adrenaline.On the nictitating membrane mepyramine caused sensitization to the actions of adrenaline, noradrenaline and tyramine, an effect similar to that of chronic preganglionic denervation. Diphenhydramine enhanced the action of noradrenaline more than that of adrenaline and had little effect on tyramine action, giving a sensitization which bears a greater resemblance to the type caused by cocaine or chronic postganglionic denervation.It is suggested that two distinct mechanisms are required to account for the phenomena of sensitization.