Abstract: The contractile responses of the rabbit aorta strips to ephedrine can be blocked by pretreatment with dibenzyline. The presence of a high concentration of ephedrine during exposure of the strips to dibenzyline resulted in "self-protection" for ephedrine itself and "cross-protection" for adrenaline or noradrenaline, but high concentrations of adrenaline or noradrenaline gave no "cross-protection" for ephedrine. Our experiments suggest that the excitatory action of ephedrine on rabbit aorta strips depends upon the presence of free adrenergic receptors and also some other receptors of undetermined nature.