Pregnant Wistar rats were injected subcutaneously with trypan blue and the heart was studied in 206 embryos removed at daily intervals from ten and one-half to fourteen and one-half days of gestation.

Cardiac anomalies appeared to be largely due either to malpositioning of the atria consequent upon an abnormal looping of the bulbo-ventricular loop, or to a decreased formation of cardiac jelly.

The hearts of a further 46 embryos at ten and one-half days were studied histochemically and 40 of these (24 experimental and 16 control) were stained with Best's carmine. Glycogen granules were found in the hearts of experimental embryos but not in the hearts of control embryos.

It is suggested that trypan blue may act directly on the cells of the myoepicardium of the developing rat heart, causing a precocious transition to the biochemical characteristics of the adult tissue with a consequent decrease in the amount of cardiac jelly formed. The abnormal looping of the bulbo-ventricular part of the cardiac tube might also be consequent upon such a biochemical change.