Solutions of NaCNS at different molarities were injected into batches of White Leghorn eggs after 1, 2, 3 and 4 days of incubation; controls were prepared by injection of NaCl solutions at the same molarities. The chicks were killed after 19 days of incubation. The research has brought to the following results: 1) The in ovo injection of thiocyanate produces no peculiar malformations, but a general inhibition of growth. 2) The solutions of NaCNS at concentration higher than 0,4 M are lethal for all individuals, from the seventh day of incubation onward. The 0,4 M and 0,2 M solutions produce a higher percentage of mortality than that of the controls; this difference in mortality is significant only in those batches of eggs which had been injected after one and three days of incubation. 3) In the treated embryos, ponderal and metrical analysis have made evident a significant decrease of weight and length, compared with those of controls. 4) The NaCNS has raised the average percentage of shrunken phalanxes of the hind limbs, among the treated embryos.