In the late 1970s and early 1980s several lines of investigation in the fields of body fluid homeostasis regulation and hypertension converged to support the existence of a circulating factor with both natriuretic and blood pressure elevating activities. The apparent mechanism of action of this putative hormone was inhibition of the Na pump (Na,K-ATPase), a mechanism held in common with the cardiac glycoside class of drugs. This similarity led to the use of the terms endogenous “ouabain-like” or “digitalis-like” when reference to this putative hormone was made.