Mechanical gastric distension induces a dorsal aortic pressor response in rainbow trout (Oncorhynchus mykiss) with no change in gastrointestinal blood flow. To elucidate what role chemical stimuli from the digested food has on the postprandial cardiovascular response, a new method was developed to investigate the contribution of individual nutrient components. Three, pre-digested experimental diets were injected directly into the proximal intestine of rainbow trout and cardiac output (CO), gut blood flow (Qcma), heart rate (HR) as well as stroke volume (SV) were recorded. Specific dynamic action (SDA) was estimated by measuring oxygen consumption. When a balanced diet (50% protein, 25% fat, 15% carbohydrate) was injected, Qcma and CO increased within 1 h by 45% and 27%, respectively. The response to a high protein diet (70% protein, 5% fat, 15% carbohydrate) was quantitatively similar but delayed, with a maximal blood flow response after 2 h. With a high lipid diet (60% fat, 15% protein, 15% carbohydrate) the peak increase in Qcma by 22% occured after 30 min and thereafter declined rapidly. The SDA response (19%) to the balanced diet was temporally matched with the hyperemia. With a high-protein diet the response is delayed and enlarged (34%) compared to the balanced diet. The high-lipid diet gave no significant SDA response. We conclude that the chemical composition of the food influences the postprandial hyperemia and the SDA, such that the components appear to work in a synergistic fashion. The present results also demonstrate that both redistribution of blood flow and an overall increase in CO contribute to the postprandial increase in gut blood flow in this species.