84 Elevated plasma concentration ([PL]) of Atrial Natriuretic Peptide (ANP) is a prognostic marker for morbidity in chronic heart failure (CHF) patients(pts). The purpose of this study was to determine (1) the relative proportion of the more biologically active α-ANP compared to β-ANP, and (2) the effects of acute EX on PL [α-ANP] and [β-ANP] in CHF pts. 15 pts (62.1±8.0 yrs) with CHF secondary to coronary artery disease (CHF duration 5±3yrs; LVEF<50%; NYHA II/III) and 9 healthy controls (CON)(67.3±8.2 yrs) performed an SL-GXT (mod. Naughton). Within one week, pts performed a submax EX test at 40% HRmax (40%EX) and 70% HRmax (70%EX) of peak EX (PK-EX). Venous blood samples were collected before (PRE-EX) and after all EX conditions and analyzed for [α-ANP], [β-ANP], hematocrit, and hemoglobin. Changes in PL volume (PV) were calculated. The table depicts the change in PL [α-ANP] and[β-ANP] corrected for EX-induced PV shiftsTableThese data indicate an increased proportion of PL [α-ANP](1.5:1) and[β-ANP] (5.9:1) in CHF pts vs CON PRE-EX. EX induces an intensity-dependent increase in PL [α-ANP] and [β-ANP](†p<0.05 vs. CON; *p<0.05 vs. PRE-EX). Despite elevated circulating [ANP], the ANP system in CHF has reduced functional activity.