We have previously shown erythropoietin (Epo) and its receptor (Epo-R) to be present in the fetal human central nervous system (CNS), and Epo to be present in the spinal fluid of normal preterm and term infants. To investigate the cellular specificities and developmental patterns of expression of these polypeptides in the human brain-areas that have not been well researched-we designed the following study. Human brains ranging in maturity from 5 weeks post-conception to adult were preserved at the time of elective abortion, surgical removal (tubal pregnancy, or removal for temporal lobe epilepsy), or autopsy. Immunohistochemistry was used to localize Epo and Epo-R reactivity in brains of different stages of development. Astrocytes, neurons, and microglia were identified in sequential tissue sections by specific antibodies. At 5 to 6 weeks post-conception, both Epo and Epo-R localized to cells in the periventricular germinal zone. At 10 weeks post-conception, Epo immunoreactivity was present throughout the cortical wall, with the most intense immunoreactivity present in the ventricular and subventricular zones. Epo-R, in contrast, was localized primarily to the subventricular zone, with little staining evident in the ventricular zone. In late fetal brains, Epo-R reactivity was most prominent in astrocytic cells, although modest reactivity was observed in certain neuron populations. In contrast, Epo staining localized primarily to neurons in fetal brains, although a subpopulation of astrocytes was also immunoreactive. In postnatal brains, both astrocyte and neuron populations were immunoreactive with antibodies to Epo-R and Epo. From these results it is clear that Epo and its receptor are present in the developing human brain as early as 5 weeks post-conception, and each protein shows a specific distribution that changes with development. We speculate that Epo is important in neurodevelopment, and that it also plays a role in brain homeostasis later in life, functioning in an autocrine or paracrine manner.