Uncovering a surprising new function for a commonly studied hormone, Balázs Mayer and colleagues have determined that vasopressin does more than maintain fluid balance for the kidneys - it also stimulates red blood cell production. Mayer et al. speculate that drugs targeting one specific vasopressin receptor could offer a much-needed safe therapy to replenish red blood cells after traumatic injuries or chemotherapy. In mouse models, the scientists showed that vasopressin boosted red blood cell formation faster than the other well-characterized hormone, erythropoietin. The researchers narrowed in on this unexpected role for vasopressin by examining clinical data from 92 patients with central diabetes insipidus (a condition that causes deficiencies of the hormone). Of those individuals, 87% of males and 51% of females had the red blood cell deficiency anemia, compared to prevalence rates of 1.5 to 6% for males and 4.4 to 12% for females among the general United States population. The scientists further demonstrated that all three varieties of the receptor for vasopressin are present on blood-forming stem cells, but one particular form of the protein (AVPR1B) plays a predominant part in red blood cell production. Treating mice with vasopressin or a specific AVPR1B activator following injury- or radiation-induced blood loss sped up recovery from anemia for the animals. The authors say that vasopressin triggers red blood cell production on short time scales, before erythropoietin kicks in.