Several chemical and pharmacologic agents have been identified as peroxisome proliferators in rodents. Most of these compounds contain a lipophilic backbone linked to an acid moiety, generally a carboxylate. Since ibuprofen and other nonsteroidal anti-inflammatory drugs share these structural characteristics, their effects on peroxisomal beta-oxidation were examined. Ibuprofen, flurbiprofen, and indomethacin caused dose-related increases in peroxisomal beta-oxidation in cultured rat hepatocytes. The dose-response for ibuprofen and flurbiprofen was roughly equivalent to that of clofibric acid, whereas indomethacin was less active. Ibuprofen and flurbiprofen are arylpropionic acids, which are structurally similar to the aryloxyisobutyric acid clofibric acid. Indomethacin differs structurally in that the acid substitution is on an indole ring. This structural difference may be responsible for the difference in activity. Ibuprofen and clofibric acid were also compared in vivo following 2-week dietary administration to rats. Ibuprofen increased relative liver weight and peroxisomal beta-oxidation and reduced serum lipids. Clofibric acid was more active than ibuprofen in vivo, particularly with respect to induction of peroxisomal beta-oxidation (16.8-fold vs 3-fold, respectively). The difference in activity of the two compounds in vivo was not consistent with the results in vitro. The disparity in peroxisomal activity of ibuprofen in the two test systems may be related to pharmacokinetic factors which are not present in vitro.