Content of enkephalins was increased in myocardium after administration of tyramine (liberator of endogenous catecholamines). Neoepinephrine decreased content of Met-enkephalin in heart. Catecholamines circulating in blood exhibited less distinct effect on content of opioid peptides in myocardium as compared with endogenous free catecholamines. Inverse correlation was found between content of cAMP and Met-enkephalin in myocardium. All the pharmacological drugs studied could exhibit the mediatory effects on opioid peptides in heart via alterations in activity of vegetative nervous system affecting the myocardium.