A quantitative method for estimating muscle hemoglobin is described. Leg and back muscles show great range in hemoglobin content which apparently depends upon exercise and determines largely the latent muscular power. Pups of 2-3 mos. may average 100 mgm. per 100 gm. muscle tissue. At 4 mos. there may be a rise to 200 mgm. and at 6 or 7 mos. to 200-300 mgm. At puberty (8-9 mos.) there may be a sharp jump to 400-500 mgm. per 100 gm. Adults vary from 400 mgm. in a quiet house dog to 1000 mgm. in an active hunting dog. During the 1st and 2nd mos. the diaphragm shows higher values than leg muscles; near puberty it falls slightly behind. Flat muscles of the trunk (recti and pectorals) are uniformly below leg muscles. The heart is much more uniform in muscle hemoglobin content corresponding to its work. In the first few mos. the content may be 100-200 mgm. per 100 gm., rising slowly to 300 at puberty, and never going much above this in a quiet dog. Heart muscle values average 300-400 for common adults, unusual exercise pushing the values up to 700 mgm. per 100 gm. High values with cardiac hypertrophy due to chronic nephritis and emphysema are found. Normal dogs may have 25-40% of the entire weight represented as striated muscle. Representing total circulating hemoglobin as 100 gm., total muscle hemoglobin amounts to 10-80 gm. under different conditions. Muscle hemoglobin is of importance in studies of end products of hemoglobin disintegration or parent substances of mature hemoglobin, and of body pigment metabolism.