Elucidating the intracellular signaling cascades which lead to differentiation programs can be a daunting but necessary task. Even more so when the nature of the differentiating stimuli can elicit different biochemical responses yet achieve the same functional outcome. In the field of cartilage and bone regeneration the importance of the extracellular signal-regulated kinase (ERK) pathway has been a controversial issue as of late. Whether differentiation results from a soluble chemical induction or a microenvironmental cue on the cells seems to have a determining effect on the role that this pathway plays in ultimate cell fate. Here we explore the role of the ERK1/2 pathway on the mechanical induction of chondrogenesis of bone marrow mesenchymal stem cells (MSC). The cells were encapsulated in fibrin gel scaffolds and subjected to a dynamic mechanical compression stimulus previously demonstrated to induce chondrogenic differentiation of the cells with and without the addition of PD98059, a selective inhibitor for the ERK1/2 pathway. Samples were then analyzed by RT-PCR and histochemical staining for markers of both chondrogenic and osteogenic differentiation. Our results show that dynamic compression induces the chondrogenic differentiation of the cells and that inhibition of the ERK1/2 pathway completely abolishes this chondrogenic response. On the other hand, inhibition of ERK1/2 under dynamic compression augments the osteogenic response of the cells and significantly increases their expression of alkaline phosphatase (ALP), collagen type I (COLI) and osteocalcin (OCN) (P < 0.05). These results were confirmed by the histochemical staining where dynamically compressed samples show staining for sulfated glycosaminoglycans (sGAG) while the inhibited and compressed samples show no sGAG but present positive staining for microcalcifications. These results would suggest that the activation of ERK1/2 can determine the ultimate cell fate between the chondrogenic and osteogenic programs in cells stimulated under dynamic unconfined mechanical compression. Published by Elsevier Inc.