Objective— Previous studies have indicated that the hyperlipidemia and gene expression changes induced by a short-term high-fat diet (HFD) are mediated through the peroxisome proliferator-activated receptor γ coactivator (PGC)-1β, and that in vitro both PGC-1β and PGC −1α increase PPARα-mediated transcriptional activities. Here, we examined the in vivo effects of these two coactivators in potentiating the lipid lowering properties of the PPARα agonist Wy14,643 (Wy). Methods and Results— C57BL/6 mice were fed chow or HFD and transduced with adenoviruses encoding PGC-1α or PGC-1β. On chow, hepatic PGC-1β overexpression caused severe combined hyperlipidemia including elevated plasma apolipoprotein B levels. Hepatic triglyceride secretion, DGAT1, and FAT/CD36 expression were increased whereas PPARα and hepatic lipase mRNA levels were reduced. PGC-1β overexpression blunted Wy-mediated changes in expression levels of PPARα and downstream genes. Furthermore, PGC-1β did not potentiate Wy-stimulated fatty acid oxidation in primary hepatocytes. PGC-1β and PGC-1α overexpression did not alter SREBP-1c, SREBP-1c target gene expression, nor hepatic triglyceride content. On HFD, PGC-1β overexpression decreased hepatic SREBP-1c, yet increased FAS and ACCα mRNA and plasma triglyceride levels. Conclusions— Hepatic PGC-1β overexpression caused combined hyperlipidemia independent of SREBP-1c activation. Hepatic PGC-1β overexpression reduced the potentially beneficial effects of PPARα activation on gene expression. Thus, inhibition of hepatic PGC-1β may provide a therapy for treating combined hyperlipidemia.