Parabiosis has been found to modify the expected blood pressure response of rats from two strains with opposite genetic propensities for experimental hypertension. When a member from one strain was united in parabiosis with a member from the other and both were maintained on high NaCl diet, the rat from the strain ordinarily resistant to it rapidly developed hypertension, in contrast to appropriate controls from this strain. The development of hypertension in this resistant animal preceded that in its mate from the strain highly sensitive to hypertension. In the latter, both the level of hypertension and mortality were significantly less than in its control. It seems likely that the hypertension observed is the resistant parabiont was initiated in its partner from the sensitive strain. This modification in blood pressures was not observed in the absence of a high NaCl diet. Parabiosis between animals from the same strain did not alter their response. Thus, as in earlier experiences (1-4) the interaction of a nongenetic factor (NaCl) with the appropriate genetic substrate appeared to be necessary for the development of hypertension. The findings are interpreted as evidence that a transmittable humoral influence plays an important role in the pathogenesis of rat hypertension. The presence of this agent is genetically determined but, under the conditions of these experiments, it took the added stimulus of dietary NaCl to demonstrate its existence.