1. Daily creatinine excretion in the urine of normal and dystrophic mice was determined and then the carcass proteins were quantitatively extracted into soluble, myofibrillar and collagenous fractions.
2. on a live body-weight basis, total carcass protein was 15% lower in dystrophic than in normal mice. Relative to carcass weight, however, the amount of protein was significantly lower only in male dystrophic mice.
3. the myofibrillar protein fraction comprised 36.3 and 32.5% of the total protein in male and female dystrophic mice and 48.8 and 45.0% respectively in normal mice. the decrease in myofibrillar protein in dystrophic mice was accompanied by an increase in the residual collagenous fraction of proteins.
4. the rate of excretion of creatinine was strongly correlated (r = +0.98) with the myofibrillar protein mass in each mouse. This relationship was the same for both normal and dystrophic mice, each gram of myofibrillar protein being associated with 3.6 μmol of creatinine excreted/day.
5. the creatinine excretion rate is a valid index of contractile muscle mass in murine dystrophy.