The major site(s) of synthesis of plasma fibronectin is unknown. Using the isolated perfused rat liver and anti-rat fibronectin antiserum, we have measured net hepatic synthesis of fibronectin during 10-hr perfusion periods. We calculated the circulating plasma fibronectin pool of a 195-g rat with body surface area of 300 sq cm to be 3.3 mg: net hepatic synthesis of 3.75 +/- 1.0 mg/300 sq cm body surface area of the rat liver donor was seen in liver perfusions of 10 hr. When the hormones cortisol and insulin were added to the liver perfusate, net synthesis of 5.05 +/- 0.82 mg/300 sq cm was seen. Synthesis of two known acute phase reactant proteins, fibrinogen and alpha-2 acute phase globulin, was also substantially greater in the presence of cortisol and insulin. Plasma fibronectin levels measured in intact rats that had abscess produced by subcutaneous turpentine injection were 0.57 +/- 0;05 mg/ml compared to 0.38 +/- 0.02 mg/ml in normal control animals. Our studies indicate that hepatic synthesis contributes substantially to the plasma fibronectin pool and show that such synthesis is enhanced by the hormones cortisol and insulin. In the intact rat, inflammation was associated with elevated levels of plasma fibronectin.