The purpose of the present study was to evaluate the response of the diaphragm (Dia) and the alae nasi (AN) muscle to bolus injection of vasopressin (AVP). Adult male rats were anesthetized. Tracheotomy and catheterization of the femoral artery and vein were performed. The rat breathed spontaneously. Blood pressure (BP), electromyograms (EMG) of the Dia and AN were observed. Two doses of AVP (0.1 and 0.5 IU/kg b.w.) were used. Low dose of AVP produced a decrease in Dia to 96% of the control (p＞0.05) and AN to 50 % (p＜0.01). High dose of AVP evoked a reduction of the Dia to 86 % of the control (p＜0.05) and AN to 42 % of the control (p＜0.01). The results indicate that AVP produces not only modulation on cardiovascular functions but also a depressant effect on respiratory-related activities of AN muscle. Inhibition of AVP upon the AN is stronger than the Dia.