The synthesis and degradation of cyclic AMP was studied in the hearts and aortas of spontaneous hypertensive rats (SHR) and of controls of corresponding age. While no changes in phosphodiesterase activity were observed, in the aortas of SHR the basal values of adenylate cyclase as well as fluoride stimulation were doubled and the stimulatory effect of isoproterenol was reduced. As these changes in adenylate cyclase activity were observed as early as in 20-day-old rats, where no elevation of blood pressure could be found, these findings might represent one of the basic deviation rather than a secondary compensatory mechanism.