In an attempt to decrease a 10 to 15 percent vascular thrombosis rate leading to graft occlusion, low-dose human-grade heparin was studied to determine if carefully monitored intravenous therapy would increase 7-day patency in a known potent thrombosis model. In New Zealand white rabbits, the type of infusate administered intravenously, either saline (30 animals) or heparin (35 animals), was selected at random after completing a 2-mm arterial inversion graft in the femoral artery. A 72-hour infusion was used in all animals; the control group received sterile saline and the experimental group received a heparin infusion at 45 microliters per hour after a 500-unit bolus. All grafts in both groups were patent at the time of groin closure. Patency in the heparin-perfused group was 67 percent (24 of 35) as compared to 19 percent (6 of 30) in the control group (p less than 0.05) 1 week postoperatively. Scanning electron microscopy showed significantly less dense fibrin deposition and a decrease in the number of aggregated platelets in the heparin-perfused grafts. Partial tissue thromboplastin time values in the experimental group ranged between 55 and 75 seconds (control 20 to 25 seconds). We have shown that heparin, an inexpensive and readily available agent, maintains 1-week microarterial patency and results in few complications in a reliable, reproducible, and versatile thrombosis model. The clinical ramifications of using an antiplatelet agent that diminishes fibrin deposition in microsurgery are apparent.