Comparison of responses to norepinephrine (NE) and KCl of iliac arterial strips from spontaneously hypertensive rats (SHR) and three strains of normotensive rats (NR) were made in tissues obtained from 5- to 7- and 15- to 17-week-old animals. The systolic blood pressures of young rats were essentially similar while the 15- to 17-week-old SHR had markedly higher blood pressures than their normotensive counterparts. The contractility and sensitivity of strips to NE from SHR were less than those from NR in both age groups. Relaxation of NE-induced responses in normal physiological or calcium-free solutions was consistently more rapid with SHR tissues. In calcium-free solution, the fast component of responses to NE was smaller in young SHR; no difference in dependence on calcium was seen in tissues from older rats. Differences in KCl-induced responses were seen only in young animals, where reduced contractility and slower relaxation in normal physiological solutions were obtained with strips from SHR. We conclude that the iliac artery from SHR differs intrinsically from those of NR since the differences in responses were evident in young rats prior to the development of stable hypertensive state in the SHR.