We have used the method of internal splinting of the ureter (endoprothesis) for the past ten years. Because we observed partial obstruction of the lumen by incrustations and/or fibrin coagula in cases in which the splints had been in place for long periods, we conducted in vitro experiments into the problem. Under constant and variable conditions, urine was perfused through plastic tubes of different materials. Organic and inorganic urine substances and urine pH were analyzed. The materials used were examined with special scanning electron microscopic techniques, and various depositions, especially in comparisons of in vivo and in vitro conditions, were found.