1. Vasodepressor responses to prostacyclin and nitroprusside were compared in anaesthetized, spontaneously hypertensive rats of the Okamoto strain and Wistar--Kyoto controls, and also in one-kidney, one-clip hypertensive rats and unilaterally nephrectomized controls of the Sprague--Dawley strain. The responses, measured as a percentage of resting blood pressure, did not differ significantly between the hypertensive rats and the normotensive controls within each strain. 2. The effects of intravenous injections of arachidonic acid were also studied in each strain. 3. The vasodepressor effects of high doses of arachidonic acid (1 or 3 mg/kg) were much greater and more prolonged in both groups of hypertensive rats. These differences were abolished by indomethacin (2 mg/kg). 4. Comparisons between the strains showed that whereas Okamoto rats have significantly greater depressor responsiveness to nitroprusside and prostacyclin than Sprague--Dawley rats, the depressor effects of high doses of arachidonic acid (1 and 3 mg/kg) were smaller in the normotensive Wistar--Kyoto than in the Sprague--Dawley rats. 5. It is concluded that hypertensive rats have enhanced ability to transform exogenous arachidonic acid into vasodilator prostanoids. This occurs both in spontaneous hypertension and in experimental renal hypertension. However, rats of the Okamoto strain appear to have reduced ability to form prostacyclin when compared with Sprague--Dawley rats.