The consequence of a folate deficiency during organogenesis has been investigated in the rat embryo. In vitro culture of 9.5 day embryos in serum from dietary induced folate deficient rats frequently resulted in abnormal embryos. Many were growth retarded and exhibited a defect in the turning mechanism which inverts the embryo from ventrally to dorsally convex. Affected embryos displayed abnormal twisting or kinking of the neural tube. Gross anaemia was also frequently observed and the protein content of the embryos was markedly less than that of embryos grown in normal rat serum. Supplementation of the deficient serum with folic acid improved growth and greatly reduced the occurrence of deformities. It virtually eliminated the incidence of gross anaemia but only partially restored the protein content to the level observed in embryos cultured in normal rat serum. The effects of the folate deficiency could not be reversed by supplementation with multivitamins or by increasing the volume of culture serum. They were, however, eliminated by supplementing the deficient culture serum with normal rat serum. The effects could also be overcome by in vivo folate supplementation; rats which had previously been so folate deficient that culture in their serum would have resulted in malformed embryos, after restoration to a folate supplemented diet produced serum which supported completely normal embryonic growth. The results indicate that embryos undergoing organogenesis require adequate folate in order for normal growth and differentiation to take place. They also suggest that some of the embryopathic effects of maternal folate deficiency are mediated by secondary effects. These may involve complex growth or metabolic factors which can be corrected in vivo but are not readily reversed in vitro.