Recent availability of intravenous soy bean oil emulsion for clinical trials in the United States prompted infusion of intravenous diets containing a constant nitrogen level (11.7 grams/m2/day) and 13 different combinations of carbohydrate (110-2300 kcal/m2/day) and fat (0-1100 kcal/m2/day) during 34 three-day studies in 5 patients who were clinically stable after injury or operation. Urea nitrogen excretion was inversely related to carbohydrate intake (P less than 0.01) and directly related to resting metabolic rate (P less than 0.01). Fat infusion did not affect nitrogen excretion at any level of carbohydrate intake. This study suggests that, when a primary clinical goal is nitrogen conservation, carbohydrate calories should be given in amounts approximating the resting metabolic rate. Additional calories and essential fatty acids now can be safely given as intravenous fat emulsion, but fat did not affect nitrogen conservation under the conditions of this study.