A single or multiple doses of L-Homocystine administered to mice during organogenesis can aggravate the developmental disturbances caused by a single dose of VPA administered on GD 8. Whereas, VPA lowers significantly plasma FA and vitamin B12 concentrations, it has no direct impact on the homocysteine concentrations. Therefore, it is proposed that high levels of homocysteine disturb the FA, vitamin B12, and possibly methionine metabolism thus providing a favorable situation for VPA to interfere with the development of susceptible embryos.