To test the hypothesis that dietary fish oil supplementation decrease systolic blood pressure in hypertensive rats by modifying cardiovascular responsiveness to adrenergic agonists, spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) received either a corn or fish oil diet, 5% (g/kg) for 10 weeks. Mean aortic pressure was lower in fish oil treated (161 +/- 7 mm Hg (1 mmHg = 133.3 Pa)) than corn oil treated (191 +/- 6 mmHg) SHR. Although dietary fish oil supplementation decreased responsiveness to norepinephrine in isolated thoracic aorta from SHR, there was no change in cardiovascular responsiveness to the beta 1 agonist dobutamine or the alpha 1 agonist phenylephrine when these adrenergic agonists were administered in vivo. However, dietary fish oil did decrease the spontaneous basal tone in aorta from both SHR and WKY. This study provides further evidence that dietary fish oil lowers blood pressure in an animal model genetically predisposed to hypertension. However, the mechanism for this decrease in mean aortic pressure in vivo does not appear to be related to modification of cardiovascular responsiveness to alpha 1- or beta 1-adrenergic agonists and may be related to a decrease in basal vasomotor tone.