The effects of three non-steroidal anti-inflammatory drugs (NSAID), carprofen, benoxaprofen and indomethacin, on 4-hour and 24-hour carrageenan pleurisy, has been investigated. All three compounds inhibited both the oedematous and cellular components of the 4-hour pleurisy. In the 24-hour pleurisy only indomethacin affected cell accumulation and the inhibitory effect observed was selective for mononuclear cells. The volume of exudate recovered from the pleural cavity at 24 hours, in animals which had received any of the three drugs, was greater than in animals receiving vehicle. The results of a time-course experiment with indomethacin suggest that the observed effects of the NSAID may be a consequence of their inhibition of both the development and decay phases of the pleural inflammation, particularly with regard to the exudative component of the response.