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Aim: To investigate the possible roles of peroxisome proliferator-activated recep-tor α (PPARα) and the signal pathway regulating the transcription of PPARα in the cardiomyocyte differentiation course of marine embryonic stem (ES) cells in vitro. Methods: The expression of PPARa during cardiomyocyte differentiation was analyzed using both Weste blotting and immunofluorescence. Cardiac specific genes and sarcomeric proteins were evaluated when embryoid bodies were challenged with PPARα specific inhibitor GW6471 at different time courses.The phosphorylation of p38 mitogen-activated protein kinase (MAPK) was stud-ied in the differentiation process, and its specific inhibitor SB203580 was em-ployed to study the function of p38 MAPK on cardiac differentiation and the expression of PPARα. Results: The expression of PPARα was observed to be at a low level in undifferentiated ES cells and markedly induced with the appearance of beating clusters. The inhibition of PPARa by its specific inhibitor GW6471 (1 x 10-5 mol/L) significantly prevented cardiomyocyte differentiation and resulted in the reduced expression of cardiac sarcomeric proteins (ie α-actinin, troponin-T) and specific genes (ie α-MHC, MLC2v) in a time-dependent manner. In the differ-entiation course, p-p38 MAPK was maintained at a high level from d 3 followed by a decrease from d 10. The inhibition of the p38 MAPK pathway by SB203580 between d 3 and d 7 efficiently prevented cardiomyocyte differentiation and re-sulted in the capture of the upregulation of PPARα. Conclusion: Taken together,these results showed a close association between PPARα and cardiomyocyte differentiation in vitro, and p38 MAPK was partly responsible for the regulation of PPARα.