The effect of tuftsin (Thr-Lys-Pro-Arg) and its partial sequences: Thr-Lys-Pro, Lys-Pro-Arg, Thr-Lys, Lys-Pro and Pro-Arg on arterial blood pressure was studied in experiments on guinea-pigs anesthetized with urethane. The peptides were administered intravenously in a dose of 1 mg/kg. The blood pressure was measured directly in the carotid artery. Tuftsin and peptides: Thr-Lys-Pro, Thr-Lys and Pro-Arg, after longer period of latency, caused protracted rise in arterial blood pressure. Such action was not shown by Lys-Pro-Arg and Lys-Pro.