ABSTRACT Treatment of Rana pipiens embryos with adenosinetriphosphate (ATP), adenosinemonophosphate (AMP-3), adenosinediphosphate (ADP), and other diphosphorylated nucleosides, either under ‘optimum conditions’ or with an overdose, resulted in formation of neural tubes about 4 hours (at 18° C.) ahead of untreated controls. With overdoses specific side-reactions appeared that differed with the substance applied. The rate of neurulation was directly related to the number of phosphate groups of the nucleotides used; it was fastest with ATP, intermediate with ADP, and slowest with AMP-3. The other side-reactions varied specifically and consistently with the number of phosphate groups, but the intensity of these reactions was not directly proportional to the number of phosphate groups present. Overdose treatment with ATP blocked development of head and gill structures immediately following precocious neurulation. This effect was largely reversible by addition of equimolar AMP-3 to solutions. Overdose treatment with AMP-3 apparently enhanced formation of head and gill structures. Development was first blocked at the tail-bud stage. ADP, IDP, and other diphosphorylated nucleosides at similar concentrations had no adverse effects following precocious neurulation and apparently stimulated growth of the entire embryo. All of the mononucleotides specifically accelerated neural-tube formation whether applied at the 2-cell stage, during the blastula stage, or at the stage of the open neural plate.