A standardized microvascular operation was performed on vessels of almost identical size inbred rabbits. The investigation comprised two series, operations being performed alternatively on anticoagulated and non-pretreated (control) animals. Highly significant, better patency rates were found in those animals which showed per- as well as post-operatively satisfactory (therapeutic) levels according to systematically performed laboratory (trombo-) tests. By this protection a patency rate of about 2/3 was achieved after re-anastomosis of transected common femoral arteries, varying in external diameter from 0.8 to 1.2 mm.