The elastic response of the thoracic aorta to increasing steps of angiotensin was studied in chronic instrumented conscious dogs with and without parasympathetic blockade by atropine. A pressure microtransducer and two ultrasonic crystals diametrically opposed and fixed in the adventitia enabled to determine the mean and systolic-diastolic changes of pressure (P) and diameter (D). By computing these measurements two representative indexes of dynamic elastic modulus in vivo were calculated; the elastic modulus of Peterson (Ep) Ep = delta P/D.D and the incremental elastic modulus (Ei) Ei = 0.75 EP/gamma, gamma being the ratio of the thickness to the external radius. A positive correlation (p less than 0.01) was obtained between pressure and diameter variations in the presence or absence of atropine but the slope of these relationship were lower with atropine than in controls. The slope of the positive correlations observed between Peterson and incremental elastic modulus and the increase in mean arterial pressure in response to angiotensin (p less than 0.01) was higher in the presence of atropine (p less than 0.05). These observations indicate that in response to angiotensin mediated high blood pressure, the cholinergic blockade of muscarinic receptors with atropine induce a contraction and increasing rigidity of the aorta.