Peroxisome proliferators increase hepatocyte proliferation and cause liver tumors in rodents, yet the mechanism of action is not understood. Based on studies with null mice it is known that peroxisome proliferator-activated receptor-α (PPARα) is involved. There is also evidence that Kupffer cells play a central role in peroxisome proliferator-induced carcinogenesis, most likely via mechanisms involving increases in superoxide, activation of nuclear factor κB and production of tumor necrosis factor-α (TNFα). However, it is not known whether PPARα is constitutively expressed in Kupffer cells. Therefore, the expression of PPAR isoforms in rat Kupffer and parenchymal cells was examined. Kupffer cells and hepatocytes of >99% purity were isolated from rats fed either a control diet or one containing 0.1% WY-14,643 for 1 week. Protein and RNA were obtained and PPAR expression was analyzed using northern and western blots. PPARα, PPARβ and PPARγ mRNA was detected in purified hepatocytes. In Kupffer cells, mRNA encoding PPARγ was present while transcripts for PPARα and PPARβ were not detected. Immunoblots were consistent with the results found by northern analysis. Moreover, when Kupffer cells from wild-type or PPARα-null mice were treated with WY-14,643 in vitro, superoxide production was similar. Combined, these results show that PPARα is expressed in rat parenchymal cells but not in Kupffer cells. These data are consistent with the hypothesis that parenchymal cells respond to Kupffer cell-derived TNFα via mechanisms dependent on PPARα within the parenchymal cells.