Summary As a result of a teratogenic maternal folic acid deficiency, discrete changes are produced in the molecular composition of abnormally developing embryos. Furthermore, these changes continue in the embryo after the mother has been returned to a complete ration. The adverse effects on the development of new enzymatic forms appeared to be both qualitative and quantitative. Although there was a general trend for delayed and asynchronous appearance of specific molecules in the abnormally developing embryos, this was not invariably the case and some of the enzymes did appear on schedule and at or near control amounts in the experimental animals. Thus the retardation in the appearance of certain enzyme forms resulting from folic acid deficiency would appear to be selective.