Hypertension increased the responsiveness of spontaneously hypertensive rats (SHR) to exogenously infused prostacyclin (PGI2) at rates of 300 ng/kg/min and 1 μg/kg/min. This did not occur for angiotensin infusion. Substitution of a diet containing α-linolenic acid for the control diet to SHR abolition of the increased responsiveness to PGI2. Accompanying the abolition of the increased responsiveness to PGI2 was a significant increase in specific activity of two lysosomal hydrolases (N-acetylglucosaminadase and cathepsin D) within the aorta of SHR. These changes may reflect compensatory responses of the vasculature to protect the animal.