EndMT (endothelial–mesenchymal transition) is a critical process of cardiac development and disease progression. However, little is know about the signalling mechanisms that cause endothelial cells to transform into mesenchymal cells. In the present paper we show that TGF-β2 (transforming growth factor-β2) stimulates EndMT through the Smad, MEK [MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) kinase], PI3K (phosphinositide 3-kinase) and p38 MAPK signalling pathways. Inhibitors of these pathways prevent TGF-β2-induced EndMT. Furthermore, we show that all of these pathways are essential for increasing expression of the cell-adhesion-suppressing transcription factor Snail. Inhibition of Snail with siRNA (small interfering RNA) prevents TGF-β2-induced EndMT. However, overexpression of Snail is not sufficient to cause EndMT. Chemical inhibition of GSK-3β (glycogen synthase kinase-3β) allows EndMT to be induced by Snail overexpression. Expression of a mutant Snail protein that is resistant to GSK-3β-dependent inactivation also promotes EndMT. These results provide the foundation for understanding the roles of specific signalling pathways in mediating EndMT.