We determined a rate of protein egress from the pleural cavity into thoracic duct lymph in seven anesthetized sheep in whom we had made 10-ml/kg hydrothoraces containing 1% plasma protein labeled with 125I-albumin. The labeled protein left the pleural space at an average rate of 0.02 ml X kg-1 X h-1. In 25 unanesthetized sheep we injected labeled protein intravenously and collected pleural fluid and plasma at intervals up to 48 h. The radioactivity in the sheeps' plasma fell twice as rapidly as the rate observed in humans. The half time for equilibration between plasma and pleural fluid protein specific activity was approximately 6 h. In seven sheep we gave a continuous infusion of tracer protein. In six of these seven sheep we collected superficial cervical lymph. The protein concentration in the lymph was three times that of pleural fluid. The half time for protein specific activity equilibration between plasma and pleural fluid was 5 h and 3-4 h between plasma and superficial cervical lymph. The rate of protein turnover in the pleural space appears to be much slower than previously reported.