Pleurodesis with a sclerosing agent was attempted in an animal model in which a pneumothorax with an active air leak was created. Adult rabbits had a small left thoracotomy and then a 1-cm controlled cut in the lung surface. Animals served as control, or tetracycline was administered. The chest tube was removed after the air leak stopped. Animals were sacrificed after 20 days. Group A was control rabbits (6); group B had powdered tetracycline placed at thoracotomy; group C, tetracycline solution, 2 ml/kg, 25 mg/ml (4); and group D, tetracycline solution, 1 ml/kg, 50 mg/ml (10). Control rabbits showed few adhesions, and the pleura appeared to be normal by histologic examination. There was a spectrum of results with tetracycline, but with the concentrated tetracycline solution (group D) histologic examination showed uniform pleurodesis and thickening of the pleura. In group D it took no longer for the pulmonary air leak to seal than with the group A (control) rabbits. We conclude that concentrated tetracycline solution is effective in causing pleurodesis even when an active air leak is present. The lung, however, must be kept expanded so that symphysis can occur between the visceral and parietal pleura.