This study was designed to determine if chronic heart failure (CHF) results in changes in Na⁺-K⁺-ATPase properties in heart and skeletal muscles of different fiber type composition. Adult rats were randomly assigned to a control (CON) (n=8) or CHF (n=8) group. CHF was induced by ligation of the left main coronary artery. Examination of Na⁺-K⁺-ATPase activity (nmol.mg protein^-1h^-1; x±SE) 12 weeks following the ligation measured using the 3-O-methylfluorescein phosphatase assay (3-O-MFPase), indicated higher (P<0.05) levels in soleus (SOL) (250±13 vs 179±18), lower (P<0.05) levels in diaphragm (DIA) (200±12 vs. 272±27), and left ventricle (LV) (760±62 vs 992±16) in CHF compared to CON, respectively. Na⁺-K⁺-ATPase protein content (pmol/g wet wt), measured by the [³H] ouabain binding technique, was higher (P<0.05) in white gastrocnemius (WG) (166±12 vs. 135±7.6), lower (P<0.05) in SOL (193±20 vs. 260±8.6), and LV (159±10 vs. 221±10), in CHF compared to CON, respectively. Isoform content in CHF, measured by Western blot techniques, showed both increases (WG; P<0.05) and decreases (SOL; P<0.05) in ₁. For ₂, only increases (Red gastrocnemius, RG; SOL, DIA; P<0.05) occurred. The ₂ isoform was decreased (LV,SOL, RG, WG; P<0.05) in CHF while the ₁ was both increased (WG; DIA; P<0.05) and decreased (SOL; LV; P<0.05). For ₃, decreases (P<0.05) in RG were observed in CHF while no differences were found in SOL and WG between CHF and CON. It is concluded that CHF results in alterations in Na⁺-K⁺-ATPase that are muscle specific and property specific. Although decreases in Na⁺-K⁺-ATPase content would appear to explain the lower 3-O-MFPase in the LV such does not appear to be the case in skeletal muscles where a dissociation between these properties was observed.