We hypothesized that exposure to hyperbaric oxygen (HBO²) would mobilize stem/progenitor cells from the bone marrow by a nitric oxide (^.NO) dependent mechanism. The population of CD34+ cells in the peripheral circulation of humans doubled in response to a single exposure to 2.0 atmospheres absolute (ATA) O₂ for 2 hours. Over a course of twenty treatments, circulating CD34+ cells increased eight-fold, although the over-all circulating white cell count was not significantly increased. The number of colony-forming cells (CFCs) increased from 16 ± 2 to 26 ± 3 CFCs/100,000 monocytes plated. Elevations in CFCs were entirely due to the CD34+ sub-population, but increased cell growth only occurred in samples obtained immediately post-treatment. A high proportion of progeny cells express receptors for vascular endothelial growth factor-2 and for stromal derived growth factor. In mice, HBO₂ increased circulating stem cell factor by 50%, increased the number of circulating cells expressing stem cell antigen-1 and CD34 by 3.4-fold, and doubled the number of CFCs. Bone marrow ^.NO concentration increased by 1008 ± 255 nM in association with HBO₂. Stem cell mobilization did not occur in knock out mice lacking genes for endothelial .NO synthase. Moreover, pre-treatment of wild type mice with a nitric oxide (^.NO) synthase inhibitor prevented the HBO₂-induced elevation in stem cell factor and circulating stem cells. We conclude that HBO₂ mobilizes stem/progenitor cells by stimulating .NO synthesis.