The behaviour and differentiation of cultured precardiac cells in the presence of neurogenic cells were studied by means of light and electron microscopy. Inhibition of the process of cardiac differentiation may be due to cellular contact or some specific factor elaborated in the process of differentiation by the neurogenic cells. Differentiation of cardiomyocytes in vitro may be compatible with the presence and contact of neurogenic cells, since both cell populations appear in the same reaggregate. Diffused substances in the medium of the culture through neurogenic cells do not appear to produce any modifications in cardiac differentiation. These processes were examined and are discussed.