Acidosis reduces the ability of nitric oxide synthase to generate nitric oxide (NO) from L-arginine (L-arg), even if dietary intake or circulating plasma levels of L-arg are normal. During systemic acidemia, therefore, vascular perfusion in one or more organs may be compromised. Arginine is also a powerful anabolic amino acid. If dietary sources of L-arg are lower than normal, or if the production of NO is reduced even without frank acidemia, then vascular perfusion, and growth, and tissue repair are likely to be compromised. Two conditions in which acidemia is reported to occur, namely slow fetal growth in utero (acidemia is severe) and loss of bone and muscle in microgravity (acidemia is modest), are compared with respect to the accompanying alteration in the balance between acidemia and NO production.