Pulmonary oxygen toxicity is a dose-dependent effect on alveolar epithelial and endothelial cells resulting in pulmonary oedema. Any concomitant effects on systemic capillary endothelium would be expected to result in capillary leakage and an increase in the tissues' water content. Total tissue water (TTW) in different organs was therefore studied in freely moving rats exposed to 100% O2 at normobaric pressure for 24 or 48 h, and compared to air-breathing control rats. The TTW for the following tissues was measured: Trachea, left bronchus, left lung, left and right ventricle, left kidney, skin (left paw-hindlimb), skin (back of the rat), left brain, left eye and thigh muscle left side. There was a significant increase in TTW of the lung accompanied by pleural effusion after 48 h of oxygen exposure as expected in all exposed animals. There was a small increase in TTW of the paw only, and a small decrease or no change in other tissues after 24 and 48 h of exposure. We conclude that there is no evidence of systemic capillary dysfunction as measured by tissue water content after exposure to hyperoxia in a dosage causing pulmonary oedema.