Adequate requirements of energy and protein in post operative total parenteral nutrition (TPN) were determined by studying the effects of energy and protein dose on nitrogen retention at varying levels of surgical stress assessed by urinary excretion of total catecholamines. Fifty-two patients received esophagectomy (severe stress), and gastric or colorectal operations (moderate stress) fed exclusively by TPN perioperatively were divided into 6 groups according to the dose level of protein and energy; Group I: 40 kcal, 1.0 g.protein/kg.day (9 patients), Group II: 40 kcal, 1.5 g.protein/kg.day (11 patients), Group III: 40 kcal 2.0 g.protein/kg.day (8 patients), Group IV: 40 kcal, 3.0 g.protein/kg.day (7 patients), Group V: 30 kcal, 2.0 g.protein/kg.day (11 patients), and Group VI: 50 kcal, 2.0 g.protein/kg.day (6 patients). Daily nitrogen balance (NB) and urinary excretion of total catecholamines (U-CA) were determined everyday pre- and post-operatively. Significantly negative correlations between U-CA and NB were observed. Statistically significant differences among the correlations of all groups were recognized. In moderate stress, increasing dose of protein and energy improved NB, and positive NB was achieved when 1.5-2.0 g.protein/kg.day with 35-40 kcal/kg.day was provided. Protein dose exceeding 3.0 g/kg.day caused the rise of BUN. In severe stress, such as following esophagectomy, NB was not improved by increasing dose of nutrients.