Spontaneously hypertensive rats (SHR) were compared to their controls, WistarKyoto (WKY) rats, with respect to contractile responses of aortic preparations and catecholamine concentrations in aorta and superior cervical ganglion (SCG). Contractile responses of aortic preparations to norepinephrine were significantly lower in SHR than in WKY rats, but relaxation responses to isoproterenol were not different. Norepinephrine levels were elevated in SCG and aortas of SHR vs. WKY rats. The results suggest that there are important differences in vascular responses of SHR compared with WKY rats, and raise the question of whether these differences are reflective of transmitter levels and their effects on alpha-adrenergic receptor sensitivity. The results also suggest that lack of beta receptor involvement in the observed alpha receptor desensitization may be important for the development of hypertension in SHR.