The effect of age on the vasorelaxant properties of the omega-3 fatty acids, docosahexaenoic (DHA) and eicosapentaenoic (EPA), in isolated rat aortic rings were investigated in 4-, 15- and 24-month-old Fischer 344 rats. Increased aortic wall thickness was seen in both 15- and 24-month-old rats. Maximal contractile responses to norepinephrine (NE) were greater in intact and de-endothelialized (ENDO-) rings from 15- and 24-month-old rats; although increased sensitivity to NE was exhibited in 4-month-old rings. DHA- and EPA-induced (1-100 mumol/l) responses were similar between the three age groups in intact rings. Removal of the endothelium enhanced relaxant responses to both DHA and EPA in all three groups. EPA-induced (1-100 mumol/l) responses ranged from-4 to 28% for both 4- and 15- month ENDO- rings and -1 to -18% in 24-month ENDO-rings. DHA ENDO-ring responses (1-100 mumol/l) were- 4 to -13% in 4 months, -5 to -23% in 15 months, and -0.8 to -16% in 24 months. Age differences were apparent with the ENDO- ring responses to DHA and EPA. These differences were seen with greater relaxant responses to both EPA and DHA in the 4- and 15-month-old aortic rings. Increased sensitivity and a greater maximal relaxant response to acetylcholine (ACH) was noted in the 15-month-old group. Four- and 24-month rings exhibited similar sensitivity and maximal relaxant responses to ACH. However, relaxation was decreased in 24-month rings at low ACH concentrations. These findings suggest that both functional and morphological changes occur with aging. DHA- and EPA-induced responses are not altered by aging in intact rings; however, removal of the endothelium enhances their vasorelaxant properties in all three age groups. This may be related to the direct actions of DHA and EPA on vascular smooth muscle, rather than on mechanisms associated with generation of endothelium-derived relaxing factor.