Based on own studies with labeled amino acids on very young animals and on numerous investigations of different authors on fetuses and prematures the high turnover rate of proteins, carbohydrates and fats in VLBW is emphasized. Thereupon some particularities of ammonia metabolism and the difficulties of exact determination of amino acid requirement are discussed. The inadequate activity of some enzymes and the immaturity of the transport processes, further, the question of the essentiality of taurine and carnitine in VLBW are debated. Briefly some dangers which result from the immaturity of the bowel mucosa are pointed out. The knowledge of the fat metabolism in VLBW has still many gaps. Beyond all doubts the unsaturated fatty acids play an decisive role for the synthesis of prostaglandins and the other derivatives, which are synthesized by the cyclooxygenases in all tissues, especially in brain. Probably the linoleic acid is an essential fatty acid for prematures. The influence of kidney immaturity on the metabolic processes in the entire organism is shortly outlined. Finally some references to the scarcely investigated field of the maturation of the biotransformation are presented, and the risky influence of some repeatedly used drugs on the intermediary metabolism and the inhibition of enzyme induction are pointed out.