Biochemical variables have been determined in 28 patients before and after resection of carcinoma of colon or rectum. A synthetic oral diet was given for four days before operation, (0.1 gN.kg/day and 165 kJ.kg/day) and four isocaloric intravenous regimens with different amounts and proportions of amino acids were given for six days after operation. During the preoperative diet the serum urea, glucose, urate, inorganic phosphate and cholesterol concentration decreased while those of triglyceride, iron and alanine amino-transferase increased. After operation the serum triglyceride, protein, albumin, iron, TIBC and urate concentrations decreased. The serum calcium fell in all groups receiving amino acids after operation while magnesium increased in all groups except the one receiving the highest amount of amino acids. The urea concentration decreased when no amino acids were given but increased when amino acids were administered whereas the reverse situation occurred with cholesterol. The highest increase in glucose level was observed when the highest amount of amino acids were given.