In eight humans with coarctation, fresh aortic tissue was examined pharmacodynamically. In four of these patients, and in 12 additional patients, the aorta above and below the coarctation was studied morphologically and compared with eight control aortas. By in vitro stimulation with potassium (127 mM), noradrenaline (18 microM), and prostaglandin F2 alpha (28 microM), postcoarctational aortic ring preparations showed a significantly greater contractility than precoarctational rings (p less than 0.05). Volumetric analysis showed significantly more collagen (P less than 0.01) and les smooth muscle mass (p less than 0.01) in the aorta above than below the coarctation. No significant differences were found between sections from the arch and distal to the ligamentum arteriosum in the normal aortas. We conclude that the precoarctational aortic wall is more rigid than the postcoarctational wall. This may influence baroreceptors in the upper vascular bed in such a way as to tolerate a higher pressure. This would explain the preoperative proximal hypertension, the paradoxic hypertension and the frequent lack of normalization of blood pressure postoperatively.