Previous observations showed an increased total cholesterol and triglycerol content and decreased phospholipid and lecithin content in blood serum of patients with elevated kynurenine accumulation in blood serum after tryptophan loading, i.e. in the case of pyridoxal-5-phosphate (P-5-P) deficiency in the organism (Rudzite et al., 1988). The increase of saturated and the decrease of unsaturated fatty acid content were typical for serum phospholipids obtained from these patients. The present work reports further studies of lipid metabolism and phospholipid metabolism, especially in experimentally induced P-5-P deficiency.