This study investigated the effect of intravascular high intensity, low frequency catheter-delivered ultrasound on vasomotor behaviour in vivo. Catheter-delivered therapeutic ultrasound induces endothelium-independent vasodilation in vitro. To study this effect in vivo we analysed the angiograms of canine coronaries and human femoropopliteal arteries before and after ultrasound exposure. Vasomotor behaviour was studied using a computer edge detection program. We analysed the angiograms of: (1) eight canine left coronary arteries before and after ultrasound; (2) eight canine left coronaries only subjected to serial angiograms, and three canine left coronary arteries of two dogs after passing the unactivated ultrasound probe down the coronary artery infusing saline at a rate of 10-20 ml.min-1, which served as controls, and (3) eight angiographically normal human arterial sites proximal to occlusions in eight femoropopliteal arteries of patients undergoing ultrasound angioplasty for peripheral vascular disease. Vessel diameter of canine coronaries after ultrasound exposure increased from 2.3 +/- 0.3 mm to 2.8 +/- 0.3 mm (21 +/- 2.3%) (P < 0.01). There was no change in vascular diameter in controls after multiple angiograms (2.4 +/- 0.3 mm vs 2.5 +/- 0.2 mm (4.2 +/- 0.2%) (P = ns). Average vessel diameter after passage of the unactivated probe with saline infusion decreased in the left anterior descending coronary artery from 3.66 mm to 3.13 mm, and in the left circumflex coronary artery from 2.88 mm to 2.62 mm. Vessel diameter in human femoropopliteal arteries increased from 3.2 +/- 0.2 mm to 3.6 +/- 0.5 mm or by 14.3 +/- 2.9% respectively (P < 0.01) after exposure to ultrasound energy.(ABSTRACT TRUNCATED AT 250 WORDS)