Femoral artery of the rat was percutaneously damaged with ultrasound (20kHz, 20 watts, 20 sec.). The arterial wall close to the irradiation source was injured whereas the opposite wall survived. The sequence of events showed that 1 h. after the sonication the smooth muscle cells have become paralyzed and the endothelium was injured in a patchy way. These cells desquamated 1-4 hours after the injury while the smooth muscle cells became severely altered. Twelve hours after the injury the endothelium has disrupted and the media has been completely injured, but the internal elastic lamina and the intimal connective tissue were undamaged and were coated with activated platelets. This procedure to injure locally the intima may be very useful to carry on further studies on activation of platelets "in vivo" or on intimal healing.