Murine hematopoietic progenitor cells synthesize substantial amounts of histamine in response to IL-3 or calcium ionophore. They also take up extracellular histamine by an active transport system. In the present study we demonstrate that this system mediates both influx and efflux of histamine. Indeed, MR16155 and thioperamide, the two 1-13 antagonists which are most effective in inhibiting histamine uptake, likewise diminish the release of preloaded histamine from bone marrow cells. These compounds also inhibit the release of histamine which has been newly synthesized by hematopoietic progenitors in response to IL-3 or calcium ionophore, as assessed by the accumulation of the mediator inside the cells in the presence of the antagonists. The potency of different histamine receptor antagonists as inhihitors of histamine release increases with their capacity to block histamine uptake.Key words: Hematopoietic progenitor; Histamine; Bidirectional transport; Interleukin-3 Histamine uptake is temperature-dependent and partially requires sodium exchange [13]. Although we have provided evidence that this phenomenon is not mediated by classical histamine receptors, Ha receptor antagonists, such as thioperamide and MR16155, are potent inhibitors. Conversely to the polyamine transport system, which exists in numerous cell types and has been extensively studied [14,15], histamine transport seems to be restricted to astrocytes, endothelial and glial cells, and is still poorly understood [16][17][18]. It has been suggested that astrocytes could use the same means of transport for histamine uptake and release [18]. Herein, we provide evidence for a bidirectional transporter in bone marrow cells (BMC) and in the IL-3-dependent cell line FDCP1, as assessed by the finding that the inhibitors of histamine uptake likewise decrease release of preloaded histamine. We also show that this system is involved in the release of endogenous histamine which is newly synthesized in response to IL-3 or calcium ionophore since we found a decrease in histamine release accompanied by an increase in intracellular histamine contents in the presence of H3 receptor antagonists.