The vascular smooth muscle (aorta, portal vein and renal arteries) from spontaneously hypertensive rats (SHR) contained a lower level of cyclic adenosine-3’5’-monophosphate (cyclic AMP). Similar differences were observed in young SHR that had not yet developed hypertension, as compared to their normotensive controls. However, no such difference was observed in the vascular smooth muscle from the cross-bred normotensive animals. The adenyl cyclase and phosphodiesterase activities of the vascular smooth muscles from SHR was lower than the normotensive controls. It is suggested that changes in cyclic AMP metabolism occur during the process of hypertension.