The present study was undertaken to investigate effect of water extracts of the mulberry leaves (MWE) on arterial blood pressure and respiration in cats. And also studied were depressor responses to intravenously administered MWE in the animals pretreated separately with atropine(2.5 mg/kg), propranolol(2 mg/kg), dibenamine (15 mg/kg), and hexamethonium (1.5 mg/kg) in order to find out the mechanism of depressor activity of MWE. The results obtained were as follows; 1) Following intravenous administration of 0.25 ml/kg and 0.5 ml/kg of MWE into the cat the maximum depressor responses observed were 60.2±2.3 mmHg and 72.3±1.7 mmHg respectively. 2) Since depressor responses to intravenously administered MWE were partially inhibited by hexamethonium and markedly by atropine, it is strongly suggested that depressor activity of MWE mainly results from its vagal effects. 3) After administration of MWE respiratory rate was invariably increased following a short period of apnea.