The present work is divided into two parts: clinical observations and experimental investigations. The endogenic formation of nicotinic acid was decreased in all patients with ischaemic heart disease, and we have found an increased accumulation of kynurenine in blood serum after tryptophan loading in many cases - a clear indication of pyridoxal-5-phosphate (P-5-P) deficiency in the organism. The results of experimental investigations showed that L-kynurenine sulphate initiates cardiac dysrhythmias as well as dystrophic changes in cardiomyocytes. Both the clinical observations and experimental investigations point out a previously unknown pathogenetic mechanism for the ischaemic heart disease, and for bradyarrhythmia as well as myocardial cell failure, caused by P-5-P deficiency in the organism.