Glycerol formal, a fluid currently used as a drug solvent in toxicity testing has proved to be teratogenic when injected into pregnant rats. In chick embryos, employed in ovo at a final concentration of 0.001%, it induces more severe and generalized malformations (excephaly, anophtalmia or microphtalmia, coelosomia, ectopia viscerorum; beak and limbs are also frequently malformed and the heart is enlarged). Chick blastoderms developed in vitro (New's technique) and treated with the same substance show an open nerve tube, irregular somites, a hyperemic blood island and a greatly enlarged heart.