The recoveries of radioactivity in cattle urine following the intravenous administration of N tau-[14CH3]methyl histidine were essentially quantitative in 5--7 d in non-lactating cows, bulls and steers and did not change with age. 2. The N tau-methyl histidine was excreted unchanged in urine. 3. N tau-methyl histidine occurred in muscle extracts both in the free form and as a perchloric acid-soluble, acid-labile form which accounted for approximately 85% of the total non-bound N tau-methyl histidine in muscle and appeared identical to a similar component identified in muscle extracts of sheep and pigs. 4. There was probably an age-related decrease in the concentration of the acid-labile component in muscle but which did not produce a measurable change in recovery of radioactivity in urine. 5. The daily excretion of N tau-methyl histidine (E, mumol) by male cattle was highly correlated with live weight (W, kg) by the equation: E = 50 . 4 + 3 . 536 (+/- 0 . 044)W (r 0. 997). The excretions progressively decreased from 4 . 04 mumol/d per kg at 100 kg weight to 3 . 62 mumol/d per kg at 600 kg. 6. By the criterion of the rate of clearance of labelled N tau-methyl histidine from the body, the excretion of N tau-methyl histidine in urine appears to be a valid index of muscle protein breakdown in cattle.
