A two-stage project was developed to study the effect of increased venous pressure on blood flow and survival in microvascular free tissue transfers. A rabbit epigastric fasciocutaneous free flap model was used. The blood flow in the rabbit epigastric free flap is 1.07 +/- 0.06 ml/min. The average venous pressure is 8.6 +/- 1.7 cm water. A logarithmic relationship between blood flow and venous pressure was demonstrated, with a statistically significant decrease in blood flow to 20-35% of the control values when the venous pressure rose above 28-30 cm water. This study stands in support of the tolerance by free tissue transfers of pressures below this range. The results further show a positive correlation between increasing venous pressure and free flap failure.