The newest neurohormonal therapy approved for heart failure (August 2015) is sacubitril/valsartan (trade name Entresto). This medication is the first of a new family of agents (ARNI = angiotensin receptor antagonist with neprilysin inhibitor), combining the approved angiotensin receptor blocker valsartan with sacubitril, an inhibitor of neprilysin, which is a neutral endopeptidase that degrades endogenous vasoactive peptides. Treatment with sacubitril increases circulating levels of natriuretic peptides, which have been shown to facilitate natriuresis and vasodilation. Although the precise mechanisms responsible for benefit in heart failure remain unclear, sacubitril/valsartan may reduce the fluid retention and vasoconstriction that contribute to heart failure symptoms, and may also decrease apoptosis and remodeling that lead to disease progression. There is limited data about the incremental acute and long-term hemodynamic effects of composite neprilysin/angiotensin-receptor inhibitors over enalapril, and these data may provide important mechanistic insights.