1. 1. The effects of 2,4-diamino-5-( P-chlorophenyl) 6-elhyl pyrimidine and 2,4-diamino-5-(3′,4′-dichlorophenyl) 6-ethyl pyrimidine on the development of embryos of the sea urchin, Strongylocentrotus purpuratus, were studied. These two compounds are known to be folic acid antagonists. 2. 2. Cleavage was retarded in all cultures exposed to the antimetabolites. Cells of odd shapes and sizes were frequently produced in such cultures. 3. 3. The specifie activity of DNA purines was sharply reduced in embryos exposed to the two antimetabolites with either glycine-1- 14C or glycine-2- 14C as precursors. The amount of DNA synthesis was only 55–60 per cent of the normal yield. 4. 4. The specific activity of RNA purines was also markedly reduced, although in some instances less seriously than that of the DNA purines. The yield was not affected by the antimetabolites. 5. 5. In one set of experiments the specific activity of adenine and guanine from the total acid-soluble fraction was greatly reduced in the cultures containing the analogues. In two other sets of experiments, the specific activity of adenosine monophosphate was depressed to a degree commensurate with that sustained by purines of RNA in the same experimental cultures. 6. 6. The proteins isolated suffered a moderate depression of incorporation of labeled amino acid in some of the cultures exposed to the antimetabolites. 7. 7. The decrease in incorporation of label by nucleic acid purines, nucleotides, and proteins was almost invariably more in embryos exposed to 2,4-diamino-5-( P-chlorophenyl) 6-ethyl pyrimidine than in those exposed to 2,4-diamino-5-(3′,4′-dichlorophenyl) 6-ethyl pyrimidine.