The role of serotonin, catecholamines, and angiotensin in the pathogenesis of mesenteric low flow states was investigated in anesthetized dogs by measurement of blood flows with electromagnetic flow meters. During dehydration or cardiac tamponade, a disproportionate decrease in superior mesenteric artery flow was demonstrated, compared with aortic flow, but renal artery flow was relatively better maintained. Depletion of serotonin and catecholamines by pretreatment with reserpine or blocking serotonin's action with methysergide did not alter the disproportionate reduction in mesenteric flow. Disproportionate superior mesenteric artery flow during dehydration and tamponade was associated with increased levels of circulating angiotensin and virtually was abolished by bilateral nephrectomy, by inhibition of enzymatic conversion of angiotensin I to II by Bothrops nonapeptide, and by competitive inhibition of angiotensin II with 1-sar, 8-ala angiotensin II. Exogenous angiotensin administered intravenously to dogs not protected by drug treatment disproportionately decreased superior mesenteric artery flow with less effect on renal artery flow. These results are compatible with the hypothesis that increased circulating levels of angiotensin during dehydration and tamponade contribute to the disproportionate reduction in superior mesenteric artery flow in the anesthetized dog.