Summary: Small amounts of serotonin injected into the left atrium produce a hypertensive cardiogenic chemoreflex. The afferent pathway is through the vagus, while both the sympathetic and the vagus nerves participate in the efferent response. In 44 anaesthetised dogs, using flow probes, we studied the direction, extent and time sequence of the changes in aortic and regional blood flow during this hypertensive chemoreflex. After serotonin mean aortic pressure rose from 14.1 to 25.9 kPa. Coinciding with the early phase of hypertension, mean cardiac output decreased from 1.95 to 1.60 litre·min-1 which resulted in a significant increase in calculated systemic resistance (+49%). After the transient initial decline there was a rapid increase in cardiac output that virtually doubled in the following 10 s. Three types of organ system dependent flow patterns are readily distinguishable. Whereas no reflex vasoconstriction is detectable in the coronary arteries, both the renal and the mesenteric vascular circuits undergo profound reduction in blood flow. At rest flow in the left anterior descending coronary artery averaged 30 cm3·min-1 and peak flow achieved during the reflex increased to 164 cm3·min-1. Control renal and mesenteric flow averaged 170 and 195 cm3·min-1 respectively. At the time of maximal reflex vasoconstriction flow was reduced to 20 and 15 cm3·min-1 respectively resulting in a 13 and 18 fold increase in calculated resistance. Between the extremes represented by the non-responding coronary circulation and marked response in renal and mesenteric circulations, an immediate but minor vasoconstriction was also observed in the regional circuits of the carotid and femoral arteries. The observed reflexly mediated adjustments of flow resemble the response patterns that characterise flight or fight reaction and may represent an important type of survival reflex.