The effects of essential fatty acid deficiency (EFAD) on vascular reactivity to vasoconstrictor stimuli were studied in rat autoperfused hindquarters. Weanling male Sprague-Dawley rats (aged 21 days) were fed diets containing 8% (weight/weight) of stearax plus 2% safflower oil (control diet) or 10% stearax (EFAD diet) for 8 weeks. There was no difference in systemic blood pressure or body weight between the two groups. Basal production of immunoreactive 6-keto-PGF1αby aortic segments was much less in EFAD aortae than in control aortae. In contrast, immunoreactive 6-keto-PGF1αproduced by incubating aortic segments with exogenous arachidonic acid (12 μmol/l) was much greater in EFAD aortae than in control aortae. Moreover, conversion of [14C]-arachidonate to [14C]-6-keto-PGF1αwas more pronounced in EFAD aortae than in control aortae. Vasoconstrictor responses to noradrenaline (0.01–1.0 μmol/l) and angiotensin II (0.001–1.0 μmol/l) infused into the blood perfused hindquarters were then examined. The rats on the EFAD diet were more sensitive to both noradrenaline and angiotensin II than rats on the control diet (P< 0.05, two-way ANOVA). Thus, a deficiency of essential fatty acids can lead to increased vascular sensitivity to vasoconstrictor stimuli. Deficiency of arachidonic acid in phospholipid stores is also accompanied by augmented cyclo-oxygenase activity in the vessel wall, similar to that observed previously in spontaneously hypertensive rats (SHR) and rats with one kidney renovascular hypertension.