It is shown in acute experiments in cats that byphasic arterial pressure changes occurred due to action of two heterodirectional and equivalent humoral stimuli. Magnitudes of both initial depressor and following pressor phases were authentically less than the ones due to separate action of those stimuli. It is noticed that the character of sum reaction depends neither on action mechanisms of vasoactive agents in the blood circulation system nor differences in the latent period of the effect of those drugs, but connected with intensiveness of stimuli. Predominance of depressor phase magnitude was shown to observe due to a rise of stimuli intensiveness down to complete disappearance of pressor reaction.
