Nitric oxide (NO) is a free radical that functions as a cell signaling molecule but at high concentrations can be toxic. It is formed from arginine, which is consumed by the mouse blastocyst, but its effect on early embryo development has been little studied. In this study, the role of NO in mouse preimplantation development has been examined in terms of developmental rate and oxidative metabolism. Zygotes were cultured in one of four media; potassium simplex optimization medium (KSOM), KSOM with amino acids (KSOMaa), KSOM without glutamine (KSOM-glut), or KSOM with 0.5 mM arginine (KSOMarg) ± l-NAME (a specific inhibitor of NO production). End points were Day 4 blastocyst rates, cell counts determined using bisbenzimide and oxygen consumption. In KSOM and KSOM-glut, the blastocyst rate was decreased by 1 mM l-NAME from 50.2% ± 3.1% and 37.4% ± 4.5% to 6% ± 3% and 0%, respectively. In KSOMaa, cavitation rates were unaltered but the blastocysts contained fewer cells (P < 0.001). Blastocysts cultured in KSOM and KSOM-glut consumed significantly more oxygen than those cultured in KSOMaa (P < 0.001 and P < 0.05, respectively). However, the addition of 0.1 mM or 1 mM l-NAME to KSOMaa significantly increased the amount of oxygen consumed (P < 0.05 and P < 0.001, respectively). The data suggest a physiological role for NO in mouse preimplantation metabolism and development. One possibility is that NO may limit oxygen consumption at the blastocyst stage at the level of mitochondrial cytochrome c oxidase.