The essential amino acid leucine has been described to specifically activate signaling pathways leading to the activation of the translational machinery and the increase of total protein synthesis. Regulation of type I collagen production by hepatic stellate cells (HSC) is a multistep process involving transcriptional and post-transcriptional mechanisms. In the present work we studied the effect of leucine on translation regulation of collagen α1(I) production in HSC and the signaling pathways involved. Treatment of HSC with 5 mM leucine did not alter half-life or steady state levels of procollagen α1(I) mRNA, but caused an increase in procollagen α1(I) protein that correlated with changes of components involved in translational regulation, like enhanced 4E-BP1, Mnk-1, and eIF4E phosphorylation. Leucine also induced mTOR, ERK, and Akt phosphorylation in HSC, without affecting p38 and JNK activation. Pre-treatment of HSC with PD098059, wortmannin, or rapamycin prevented the profibrogenic action of leucine due to the inhibition of different molecular mechanisms. These results suggest leucine is a profibrogenic agent for HSC, activating signaling pathways that lead to an enhancement of collagen α1(I) production through translational regulation. J. Cell. Physiol. 209: 580–586, 2006. © 2006 Wiley-Liss, Inc.