The experimental behavior of a 1-mm internal diameter (i.d.) polytetrafluoroethylene (PTFE) microprosthesis, as a substitute for an abdominal aortic segment in the rat, was reviewed. Fifty Wistar rats were divided into four groups: Group I--12 rats with autotransplant of an abdominal aortic segment (AAS); Group II--12 rats with allotransplant of an AAS obtained from Long-Evans rats; Group III--12 rats with xenotransplant of an AAS taken from rabbit femoral arteries; and Group IV--14 rats with substitution of an AAS by a 1-mm i.d. PTFE microprosthesis. The rats were sacrificed at different time intervals ranging from five to 360 days, with previous aortography. In Group I, there was a 100 percent patency at a mean of 152.41 days; in Group II, a 91.6 percent patency at a mean of 100.08 days; in Group III, an 83.3 percent patency with a 75 percent aneurysmal dilation at a mean of 107.58 days; in Group IV, a 71.42 percent patency with two anastomotic aneurysms at a mean of 105 days (P less than 0.05, chi square) between Groups I and IV, autotransplant vs. PTFE). The 1-mm PTFE microprosthesis placed in the arterial system of the rat proved to be a reliable alternative for microvascular substitution.