Experimental coarctation of the aorta has been produced in rats. Animals with the clip above both renal arteries have been compared with animals with the clip below both renal arteries and with dummy operated rats. The fore- and hindlimb blood pressures, heart weights and perfusion pressure, and norepinephrine responses of the isolated perfused hindquarters have been measured.Only the animals with a clip above the renal arteries developed hypertension and hypertrophy of the heart. The increase in blood pressure was progressive, requiring about 10 days to reach hypertensive levels. Such animals showed an increase in the norepinephrine responses and in the perfusion pressure of hindlimb blood vessels when compared with either of the other two groups. The differences are statistically significant. In rats with clips above the renal arteries, there is a highly significant regression line of hindlimb norepinephrine responses on hindlimb perfusion pressure and of either of these values on heart weight.It is considered: (a) that the hypertension in experimental coarctation of the aorta is not purely mechanical and the kidneys are essential for the development of this type of hypertension; and (b) that the increase in the norepinephrine responses and in the perfusion pressure found in the hindquarter vascular bed of rats with coarctation of the aorta above the renal arteries is not secondary to the hypertension as such, but in some way is related to the mechanism that produces the hypertension in experimental coarctation of the aorta.