The objective of the investigation was to assemble more knowledge of catabolic processes after cardiosurgery. In 14 patients after 6-hour intervals following an aortocoronary bypass the total production of urea nitrogen, Bistrian's catabolic index and the sodium:potassium ratio in urine was assessed. The urea nitrogen production varied within a range of 5.4-9.5 g per 12 hours. These values when calculated per 24 hours are 94-108 g of protein. The catabolic index was in the range of 5.8-11.2, i.e. within the zone of intense stress. Also the values of the Na/K ratio (1.0-1.9) suggested intense stress. The authors discuss the causes and possibilities to relieve the stress response.