Abstract 1. 1.|The hypothesis has been examined that an alteration in the composition or content of myosin light chains may be responsible for the lower myofibrillar ATPase activity reported in hypertrophied and failing hearts. Left ventricular hypertrophy was produced in calves by aortic coarctation and animals were studied 1 month after banding. 2. 2.|In agreement with previous work, Ca 2+- and K +(EDTA)-stimulated myosin ATPase activity was usually reduced in hypertrophied ventricles. 3. 3.|Normal calf cardiac myosin contained two light subunits with molecular weights of 26 000 and 17 000 as estimated by sodium dodecylsulfate-acrylamide gel electrophoresis. The molecular weights of light chains and the proportions of light chains in myosin from hypertrophic ventricles were the same as in myosin from normal hearts. 4. 4.|The amino acid composition and the two-dimensional “maps” of tryptic digests of myosin light chains from hypertrophic and normal ventricles were the same. 5. 5.|We find no evidence that a change in myosin light chain content or chemical composition is responsible for the lower myosin ATPase activity of hypertrophic hearts.