The validity of the urinary excretion of N tau-methyl histidine (N tau-MH) by pigs as an index of muscle protein breakdown in vivo was tested using the criterion of the rate of recovery of radioactivity in urine following an intravenous dose of N tau-[14CH3]methyl histidine. 2. Urinary recoveries of radioactivity from five animals were less than 21% of dose in 7 d after which the daily recovery was less than 0 . 3% per day. 3. The incomplete recoveries of radioactivity were associated with the presence in muscle of a large pool of non-protein-bound N tau-MH, the concentration of which increased with age. 4. The N tau-MH in this pool was present as free N tau-MH and in a dipeptide which constituted more than 90% of the total non-protein-bound N tau-MH. The contribution of the peptide increased with age, reaching 99. 8% in older animals. 5. The pool of non-protein-bound N tau-MH was maintained and increased in both established and newly accreted tissue by retention of some of the N tau-MH released by muscle protein breakdown, only a proportion of which was therefore available for excretion. Hence, the urinary excretion of N tau-MH is not a valid index of muscle protein breakdown in pigs.
