After inhibition of spontaneous tone, the contraction of spirally cut strips of aorta in response to norepinephrine was studied quantitatively in normotensive rats and in 4 different types of hypertensive rats. In no instance did the aortas of hypertensive rats show hyper-responsiveness to norepinephrine. The majority of responses were entirely normal; in a few instances there was decreased responsiveness. In vivo studies which fail to consider the critical effect of an altered “base-line” state in hypertension may demonstrate “hyper-responsiveness” which is more apparent than real.