In order to investigate the interactions of cellular elements and protein on constantly deforming (fatiguing) blood contact surfaces, a series of ex vivo canine arteriovenous shunt experiments were conducted. While fresh blood was flowing through Silastic tubing shunts, portions of the tubing were stretched 20 to 60% at a frequency of 20 to 90 cycles per minute for 10 to 90 min. The surfaces of the tubing that were stretched were compared with control tubing surfaces taken from the arterial side of the test segment using scanning electron microscopy and interference phase contrast microscopy. Approximately the same number of platelets were deposited on the stretched as on the unstretched portions of the tubing in the ten minute experiments. On the control portions of the tubing, the platelets were deposited singly and uniformly in what appeared to be a fairly inactivated state. On the stretched tubing, more pseudopod extension and aggregation was observed. In these preliminary experiments, no differences were noted as a function of frequency of stretch. As the blood contact time and the percent stretch were increased, only nonuniform, scattered aggregations of platelets, and platelets mingled with fibrin were seen. Significant numbers of spread white blood cells were observed on many of the segments of Silastic tubing stretched 20% for as short a time as 15 min. Granulocytes have occasionally been reported on less hemocompatible biomaterials after exposure to canine blood. This helps to confirm that substrate stretching of 20-60% had an adverse effect on the blood compatibility of the Siliastic tubing.