Selected aspects of total parenteral nutrition in man are reviewed with emphasis on carbohydrate, lipid, and hormonal interactions. In an experiment to test for the essentiality of a specific amino acid, a normal adult man received all major and minor nutrients, except lipids, by intravenous alimentation for 48 days. A chemical deficiency of essential fatty acids without clinical symptoms was promptly observed in four plasma lipid fractions and the phospholipid fraction of erythrocytes. With the glucose providing 2,600 kcal/day, blood glucose remained normal, but a hypertriglyceridemia was observed. Plasma insulin rose and cortisol fell. To meet the essential fatty acid and energy requirements of man, critical experiments are needed to define the safe utilization of lipid emulsions during prolonged intravenous nutrition.