The relationship between lymph flow, lymph node arterial and venous perfusion pressure and lymph node resistance have been studied in an in vivo isolated canine iliac lymph node perfused through an afferent lymphatic with heparinized canine plasma. The relationship between the rate of perfusion and perfusion pressure across the node was linear but the calculated resistance of the node decreased as the rate of perfusion increased. In nine dogs the mean resistance to lymph flows less than 0.1 ml/min was 180 mmHg/ml min, but 68 mmHg/ml min to rates of perfusion above 1.0 ml/min. An increase of venous pressure in the veins draining the node increased the node's resistance by 8.6 mmHg/ml min for each 10 mmHg increase of venous pressure. The effect on node resistance of an increase of venous pressure was greater at low rates of perfusion. A decrease of arterial pressure in the arteries supplying the node reduced the node's resistance by 2 mmHg/ml min for each 10 mmHg decrease of arterial pressure. Increases of arterial pressure had an opposite effect of a similar magnitude. The effect on node resistance of a change of arterial pressure in either direction was greater at low rates of perfusion.