AbstractOnce hematopoiesis is established in the bone marrow, a continuous egress of hematopoietic stem cells (HSCs) to the periphery occurs at a low frequency. It has been proposed that this phenomenon is part of a regenerative homeostatic mechanism that ensures the maintenance of hematopoiesis through the life of the individual. The administration of certain cytotoxic drugs or cytokines can enhance the mobilization of hematopoietic progenitors to the periphery. During the past 15 years, granulocyte‐colony stimulating factor (G‐CSF) has been used as a standard cytokine for mobilization protocols in experimental models and in humans. Despite extensive efforts by multiple groups, a definitive mechanism explaining its role in mobilization has not been provided. In a recent paper, Katayama et al.,1 through a series of clever associations supported by well‐defined experimental systems, proposed that signals through the sympathetic nervous system modify the activity of the hematopoietic niche, acting as regulators of the mobilization of hematopoietic progenitors. This surprising finding adds a new level of complexity to the cellular milieu responsible for generation and maintenance of the hematopoietic niche. BioEssays 28: 687–691, 2006. © 2006 Wiley Periodicals, Inc.