The literature on biochemical changes in open cardiac surgery with especial reference to plasma protein and amino acids has been reviewed. The changes that take place in plasma proteins during oxygenation of blood of open cardiac surgery have been investigated with emphasis on denaturation. This phenomena has been examined by the assay of blood and plasma sulfhydryl groups by different reagents and also viscosity and turbidity changes. The limitations of the two latter investigations has been discussed. Haemolytic changes in plasma have been examined and the related importance of sulfhydryl groups discussed. Plasma protein changes have been investigated by methods depending on net charge, shape, size, and molecular weight of individual molecules. Although adrenal cortical response to open cardiac surgery has been investigated it has been pertinent to reinvestigate adrenal cortical secretion pari passu with amino acid data from blood and urine samples of patients not only because of their metabolic associations, but also because of the opportunity of comparing surgical patients receiving supplementary haydrocortisone throughout open cardiac surgery with those not supplemented - the latter therefore depending on their own intrinsic secretion during surgical stress. A review of relevant literature has accordingly been presented with emphasis on the difficulties involved in ACTH assay of plasma from surgical patients, and the practical advantages of 17-hydroxycorticostereid assay in lieu of ACTH assays. The plasma and urinary amino acid increase have been examined together in detail comparing not only the increase in each amino acid at each operation but also comparing the increase of an amino acid in all the operations investigated thus obtaining mean values of each amino acid at regular intervals of time throughout each perfusion.