The effects of amino acids and ammonium on the postimplantation development of cultured preimplantation mouse zygotes were assessed. Development after transfer revealed that the mouse embryo undergoes a switch in nitrogen requirements during the preimplantation period. Although Eagle's nonessential amino acids and glutamine supported the highest implantation and fetal development rates per embryo transferred when zygotes were cultured for 48 h, by 93 h of culture the highest implantation rate was observed when all 20 amino acids were in the culture medium. Furthermore, fetal development per implantation at 69 and 93 h of culture was increased only in the presence of essential amino acids without glutamine. The beneficial effects of amino acids on postimplantation development when embryos were cultured for 4 days required that the medium be renewed after 48 h (at the 6-8-cell stage) to alleviate the build-up of ammonium. Ammonium was shown to induce fetal retardation and exencephaly in a time- and concentration-dependent manner. Renewal of amino-acid-free culture medium reduced fetal mass, providing indirect evidence for the production of an embryo-derived growth factor capable of stimulating postimplantation development. These data demonstrate that inclusion of amino acids in the culture medium for preimplantation embryos significantly increases postimplantation development the preimplantation mouse embryo changes its nitrogen requirement as development proceeds, nonessential amino acids increase the implantation rate while the essential amino acids enhance fetal development, and ammonium in the medium retards fetal development and induces the neural tube defect exencephaly.