1. Differences in the ability of metabolically-inert peroxisome proliferators (perfluoro-n-decanoic acid (PFDA, C10), perfluoro-n-octanoic acid (PFOA, C8), perfluorooctane sulphonic acid (PFOS, C8) and 1-H,1-H-pentadecafluoro-n-octanol (PFOL, C8)) to induce three forms of hepatic microsomal carboxylesterase, namely RL1, RL2 and RH1, in the male rat were studied by measuring changes in hydrolytic activities towards p-nitrophenyl acetate (PNPA), isocarboxazid (ISOC) and butanilicaine (BUTA), which are thought to be specific substrates for RL1, RL2 and RH1, respectively, and by evaluating changes in the contents of the three isozymes by radial immunodiffusion assay with specific antibodies. 2. The administration of PFDA rather specifically decreases PNPA hydrolase activity and RL1 content. On the other hand, PFOA, PFOS and PFOL markedly increase all three hydrolase activities and the content of all three isozymes (except RH1 in the case of PFOA, where the increase was not statistically significant). 3. The correlations between hydrolase activities and isozyme contents supported specificity of the three substrates, with the exception that the content of the predominant isozyme, RL2, showed a higher correlation with BUTA hydrolase activity than with ISOC hydrolase activity. 4. In conclusion, we have demonstrated that metabolically-inert perfluorinated fatty acids induce hepatic microsomal carboxylesterase isozymes, as determined by radial immunodiffusion analysis using specific antibodies. This is the first report that perfluorinated fatty acid affect carboxylesterase isozymes in rat liver microsomes, and is indicative of the importance of peroxisome proliferators in hepatic metabolism of xenobiotics. Further work is needed to determine the regulatory mechanisms involved.