1. To establish whether 3-methylhistidine in food is quantitatively excreted in the urine, five normal adults on a vegetarian diet were given known amounts of 3-methylhistidine in meat or fish. The mean cumulative 5-day increment in 3-methylhistidine excretion in non-hydrolysed urine accounted for an average of 90% of the 3-methylhistidine given during the first 3 days of this period. 2. To define the variation in urinary 3-methylhistidine on a constant mixed intake, the daily 3-methylhistidine excretion was measured for 6 days in four patients on a metabolic ‘balance’ diet. The mean daily variation was less than 10%. 3. The results show that under the conditions of this study the increase in urinary 3-methylhistidine above basal levels can be quantitatively accounted for by the 3-methylhistidine content of additional food; in subjects on a constant diet it should be possible to detect relatively small changes in 3-methylhistidine excretion.