Three types of vasorelaxants were used to test the responses of canine veins isolated from 13 different sites: isoproterenol, papaverine and nitroglycerin. Strips were preconstricted with methoxamine (5 × 10–6–10–5 M), KC1 (50 mM) and PGF2α (1 µg/ml), and were relaxed by cumulative addition of relaxants. Isoproterenol caused more than 80% relaxation after preconstriction with methoxamine in cephalic, external jugular, azygos, renal, femoral, lateral saphenous veins and the supradiaphragmatic and infrarenal portions of the inferior vena cava, all of which are veins of the body wall. Pulmonary and splenic veins also showed marked relaxation with isoproterenol. However, maximal relaxation responses of portal, mesenteric veins and segment C of the inferior vena cava (a portion between liver and renal veins), which are embryologically related to the digestive tube, were less than 30%. Similar regional differences in the relaxation responses to isoproterenol were obtained after preconstriction with KC1 or PGF2α. Papaverine and nitroglycerin caused nearly uniform relaxation in all veins, although relaxations of segment C of the inferior vena cava were slightly less than those of other veins. These results indicated that there is a regional difference in the relaxation responses of the canine venous system to isoproterenol, and such a difference may be related to its embr