1. The rate of whole-body nitrogen flux; protein synthesis and protein breakdown were measured in patients with colorectal cancer (Dukes A—C) just before and 12 weeks after surgical removal of the tumour. The rates were determined from the urinary excretion of 15 N in ammonia and in urea over a 9 h period after an oral dose of [ 15 N]glycine. 2. The food intake during the 2 study days was identical for individual patients. The amount each received was determined from measurement of their intake of food ad libitum on the day preceding the pre-operative study and was consumed in six equal portions every 2 h during the experimental period. 3. No significant differences in the rates of nitrogen flux, protein synthesis and protein breakdown were found before and after tumour resection, whether calculated from the excretion of 15 N in ammonia or in urea. Some changes in flux, both increases and decreases, were observed in individual patients after tumour removal but these could not be related to classification of the tumour, or to the presence of pre-operative anorexia or weight loss. 4. The results suggest that the primary tumour itself does not alter the overall rate of protein metabolism in the whole body.