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.

1. Four normal adults were given [ 15 N]-glycine in a single dose either orally or intravenously. Rates of whole-body protein turnover were estimated from the excretion of 15 N in ammonia and in urea during the following 9 h. The rate derived from urea took account of the [ 15 N]urea retained in body water. 2. In postabsorptive subjects the rates of protein synthesis given by ammonia were equal to those from urea, when the isotope was given orally, but lower when an intravenous dose was given. 3. In subjects receiving equal portions of food every 2 h rates of synthesis calculated from ammonia were much lower than those from urea whether an oral or intravenous isotope was given. Comparison of rates obtained during the post-absorptive and absorptive periods indicated regulation by food intake primarily of synthesis when measurements were made on urea, but regulation primarily of breakdown when measurements were made on ammonia. 4. These inconsistencies suggest that changes in protein metabolism might be assessed better by correlating results given by different end-products, and it is suggested that the mean value given by urea and ammonia will be useful for this purpose.