Arterial smooth muscle cells from rabbit aortic media were grown in first subcultures on hydrophilized and collagen-coated silicone membranes which were then subjected to directional cyclic stretches and relaxations at a frequency of 50 times/min. The membranes were stretched 2, 5 and 10% beyond their resting length. Cells on unstretched and stationary membranes in the same chamber served as controls. The cells which were stretched with an amplitude of 2% remained in random orientation after 14 days of continuously performed cyclic stretching. The cells which were stretched 5% for 12 days orientated at an angle of 61 +/- 9 degrees to the direction of stretching, while the cells which were stretched with an amplitude of 10% for 6 days orientated at an angle of 76 +/- 5 degrees. The cells on the stationary and unstretched membranes remained in random orientation. We were able to confirm that the angle of orientation is reversible, i.e. preorientated cells changed their orientation during application of another stretching amplitude. The results suggest that stretching of the artery wall by blood pulsation may be a factor influencing the orientation of smooth muscle cells within the media of the artery wall and of those smooth muscle cells which proliferate into the subendothelial space after mechanical injury of the endothelium or electrical stimulation of the artery wall. An apparatus is presented which produces cyclic and directional mechanical stimuli similar to those which may occur in the artery wall.