Based on animal studies, the amino acid 3-methylhistidine, a component of actin and myosin, is not reutilized for protein synthesis following the breakdown of muscle protein but is quantitatively excreted in the urine and should thus provide an index of the rate of muscle protein breakdown. The urinary outputs of total N, urea N, 3-methylhistidine, 1-methylhistidine, and creatinine are reported for three obese subjects during a 20-day starvation period. The excretion of 3-methylhistidine decreased progressively and on day 20 was 34% less than on day 3. This reduction was proportionally similar to the reduction in urinary total N output but was much greater than the 15% decrease in creatinine excretion and in calculated loss of muscle mass. Assuming that tissues other than muscle are not significant sources of urinary 3-methylhistidine, these findings suggest that the reduced output of urinary N is due to an adaptive decrease in the rate of catabolism of muscle proteins as starvation progressed.