Two types of spongy polyurethane-polydimethylsiloxane blend (Cardiothane 51, Kontron Instruments, Inc., Everett, Mass.) vascular grafts with an internal diameter of 1.5 mm were fabricated by a spray, phase-inversion technique. Low-porosity grafts with hydraulic permeability of 2.7 +/- 0.4 ml/min per square centimeter and medium-porosity grafts with hydraulic permeability of 39 +/- 8 ml/min per square centimeter displayed good handling properties and suturability. Twelve straight low-porosity grafts, 17 straight medium-porosity grafts (1.5 to 2.0 cm in length), and one loop medium-porosity graft (10 cm in length) were implanted by the same surgeon end to end in the infrarenal aorta of 30 male Sprague-Dawley rats. Three months after implantation, patency was 8% for low-porosity grafts (1/12) and 76% for straight medium-porosity grafts (13/17). The loop medium-porosity graft was also patent. The sole patent low-porosity graft showed neointimal hyperplasia and incomplete endothelialization. All but one of the patent straight medium-porosity grafts showed a glistening and transparent neointima with complete endothelialization and no anastomotic hyperplasia. The loop medium-porosity graft displayed endothelialization from each anastomosis and in many islands in the middle portion of the graft, totalling 47% of the luminal surface by morphometric analysis. Thick mural thrombus, anastomotic hyperplasia, or aneurysm formation were not observed in any patent medium-porosity graft. These data indicate that in the rat aortic replacement model it is possible to achieve patency and a high degree of endothelialization in very small-diameter prostheses of appropriate porosity.