A model combining data from balance experiments with data from oxidation of nutrients demonstrating the pools of protein, carbohydrate and fat and their partition in the body was presented. Data from more than 200 experiments with growing pigs were used to fill up the "black boxes" in the model and to discuss the pattern between catabolic and anabolic processes of the different nutrients. With a ME-intake of 1.3 MJ/kg0.75 the proportion of retained protein from the protein pool varied from 50 to 75% depending on the age of animals and the protein quality. At a low protein intake and ME of 0.6 MJ/kg0.75 the utilization of protein was reduced to about 25% as a substantial part of the protein pool was oxidized. The carbohydrate group from the protein sources constituted less than 5% of the carbohydrate pool. The oxidation of carbohydrates was between 50-75% of the pool in all experiments, while the rest was used for lipogenesis. Even on a low feeding level the pattern occurred indicating a requirement for specific substances formed in the body. Lipogenesis constituted 46-96% of the fat pool depending of the amount on digested fat in the different experiments. No oxidation of fat was observed in experiments with pigs (30-90 kg LW) on a high feeding level in spite of the broad variation in the amount of digested fat, indicating a complete storage of the fat pool into body fat. In experiments with piglets (3-9 kg LW) on the same energy intake fat oxidation of about 30% occurred, probably caused by the high energy requirement of the piglets.(ABSTRACT TRUNCATED AT 250 WORDS)