The effects of pulmonary instillation of hydrochloric acid on solute flux from the lung (measured as the clearance of 99mTcDTPA) together with an index of oedema formation (the ratio of lung wet weight to lung dry weight) was measured in rats. There was a significant increase in 99mTcDTPA clearance (P less than 0.001) and also in lung wet:dry weight ratio (P less than 0.01) 4-5 h following acid challenge. There was no difference in the effects of challenge with acid of pH1 and pH2. In addition, intravenous injection of indoprofen (20mg/kg) a non-steroidal anti-inflammatory agent failed to produce any beneficial effect on the variables under study when given after acid (pH2) challenge. There was also histological evidence of an influx and sequestration of granulocytes into the lung. Despite this, the plasma concentration of thiobarbituric acid reactive material (used as an index of cell-derived, oxidant-free radical production) was significantly reduced in all acid-treated groups. These data show that hydrochloric acid will initiate a severe inflammatory response in the lung of the rat and that the non-steroidal anti-inflammatory agent indoprofen when given after the injury produced no evidence of a beneficial effect on this inflammatory response.