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The development of the dual peroxisome proliferator–activated receptor (PPAR) α/γ agonist tesaglitazar as an oral antidiabetic was recently discontinued. Here we present tumor data from a 2-year carcinogenicity study in rats given 0.3, 1, 3, and 10 μmol/kg tesaglitazar is presented with focus on the findings of subcutaneous fibrosarcomas. To investigate the mechanism for induction of fibrosarcomas, replicative DNA synthesis (immunohistochemical detection of BrdU-labeled cells) and expression of PPARγ (immunohistochemistry and reverse transcription–polymerase chain reaction) in subcutaneous adipose tissues was assessed in rats administered 1 or 10 μmol/kg for 2 weeks or 3 months. Poorly differentiated subcutaneous mesenchymal sarcomas with a predominant spindle cell appearance occurred at the highest dose level of 10 μmol/kg in both sexes, and these tumors were diagnosed as fibrosarcomas. The 10-μmol/kg dose was at or above the maximum tolerated dose and caused considerable cardiovascular mortality. Tesaglitazar stimulated DNA synthesis mainly in subcutaneous interstitial mesenchymal cells. The percentage of BrdU-labeled interstitial cells was increased at 1 and 10 μmol/kg after 2 weeks. The increase in DNA synthesis was still significant at the end of the 12-week treatment at 10 μmol/kg, the dose producing fibrosarcoma. However, at 1 μmol/kg, a dose below the no-observed-effect level for fibrosarcoma, the level of DNA synthesis was similar to control levels at 12 weeks. Immunohistochemical analyses showed no detectable PPARγ protein in the majority of BrdU-labeled interstitial mesenchymal cells in white and brown fat. This indicates that stimulation of DNA synthesis is not mediated via direct activation of PPARγ in these cells. The results suggest that the induction of rat fibrosarcoma by tesaglitazar, at exposures 100-fold above the human therapeutic exposure, may involve proliferation of undifferentiated mesenchymal cells in subcutaneous tissues. [ABSTRACT FROM AUTHOR]