Mouse embryonic stem (ES) cells are pluripotent cells that differentiate into multiple cell lineages. The commitment of ES cells into the adipocyte lineage is dependent on an early 3-day treatment with all-trans retinoic acid (RA). To characterize the molecular mechanisms underlying this process, we examined the contribution of the extracellular-signal-regulated kinase (ERK) pathway. Treatment of ES cell-derived embryoid bodies with RA resulted in a prolonged activation of the ERK pathway, but not the c-Jun N-terminal kinase, p38 mitogen-activated protein kinase or phosphoinositide 3-kinase pathways. To investigate the role of ERK activation, co-treatment of RA with PD98059, a specific inhibitor of the ERK signalling pathway, prevented both adipocyte formation and expression of the adipogenic markers, adipocyte lipid-binding protein and peroxisome-proliferator-activated receptor gamma. Furthermore, we show that ERK activation is required only during RA treatment. PD98059 does not interfere with the commitment of ES cells into other lineages, such as neurogenesis, myogenesis and cardiomyogenesis. As opposed to the controversial role of the ERK pathway in terminal differentiation, our results clearly demonstrate that this pathway is specifically required at an early stage of adipogenesis, corresponding to the RA-dependent commitment of ES cells.