The transcription factor Eomesodermin (Eomes) is involved in early embryonic patterning, but the range of cell fates that it controls as well as its mechanisms of action remain unclear. Here we show that transient expression of Eomes promotes cardiovascular fate during embryonic stem cell differentiation. Eomes also rapidly induces the expression of Mesp1, a key regulator of cardiovascular differentiation, and directly binds to regulatory sequences of Mesp1. Eomes effects are strikingly modulated by Activin signalling: high levels of Activin inhibit the promotion of cardiac mesoderm by Eomes, while they enhance Eomes-dependent endodermal specification. These results place Eomes upstream of the Mesp1-dependent programme of cardiogenesis, and at the intersection of mesodermal and endodermal specification, depending on the levels of Activin/Nodal signalling.