Autophagy dysfunction has been observed in disorders of lipid metabolism, especially in the liver. Here we report our identification of peroxisome proliferator activated receptor α (PPARα) as a key suppressor of autophagy in liver, possibly by inhibiting the activity of forkhead box protein O1 (FoxO1). This study was performed using PPARα knockout (PPARα KO) mice treated with the PPARα agonist, fenofibrate (FB, 50mg/kg/day) as compared with wildtype (WT) mice. In WT mice, FB treatment induced a significant suppression of autophagy, as indicated by reductions in Atg3, Atg4B, Atg5, Atg7, Beclin1 and accumulation of p62 (all p<0.01 vs. WT untreated mice). In contrast, no significant changes perceived in any of these autophagy makers in PPARα KO mice when treated with FB, confirming that PPARα is required for FB to suppress the autophagy pathway. We next investigated the upstream mechanism mediating the suppressed autophagy triggered by activated PPARα. Firstly, we examined the mammalian target of rapamycin (mTOR) signalling, a well-recognized mechanism regulating autophagy. Unexpectedly, our data revealed that the mTOR pathway was not involved because this pathway was inhibited during PPARα mediated suppression of autophagy. Instead, PPARα activation markedly reduced the content of FoxO1 by increasing the acetylation rather than phosphorylation of this transcription factor. As inhibition of FoxO1 activity is an important mechanism to suppress autophagy, our data suggest that FoxO1 may mediate the suppression of autophagy induced by PPARα activation. We further detected an associated decrease in the deacetylase sirtuin 1, suggesting its possible effect on the observed increase in FoxO1 acetylation. Taken together, our data indicate a possible role of the autophagy pathway in mediating the metabolic effects induced by PPARα activation. These findings may have potential implications for the use of PPARα activators for the treatment of dyslipidaemia and provide new insights into the metabolic changes during starvation.