We used a continuous 12-h infusion of Escherichia coli endotoxin (10 ng.min-1.kg-1) in 10 awake sheep equipped with a lung lymph fistula and vascular catheters to determine the time course of increased plasma tumor necrosis factor-alpha (TNF-alpha) during the infusion and a 12-h postinfusion period. Lung lymph flow increased progressively during the infusion to a peak value averaging 8.6 +/- 2.0 times the baseline flow of 6.3 +/- 1.3 g/h. During the postinfusion period, lung lymph flow remained elevated at three to four times baseline. The lymph-to-plasma protein concentration ratio was unchanged from baseline over 24 h, indicating a dramatic increase in net protein flux across pulmonary microvessels. The TNF-alpha concentration peaked early in the infusion and then declined, despite the continuing presence of endotoxin. Plasma TNF-alpha concentration increased 10-fold (0.33 +/- 0.05 ng/ml at baseline to 3.89 +/- 0.78 ng/ml peak) 2 h into the endotoxin infusion. At the end of the endotoxin infusion, plasma TNF-alpha had decreased to 1.16 +/- 0.19 ng/ml. The circulating TNF-alpha concentration did not correlate with pathophysiology or outcome in these sheep.