Now there is a growing evidence that erythropoietin receptors (Epo-R) are present also in some nonhematopoietic tissues such as endothelial cells and fetal cells of neural origin, although the physiological role of Epo-R at these sites is unclear. There are some speculations that Epo-R may be expressed on cells only in the developing CNS. The objective of this study was to determine whether Epo-R mRNA may be expressed in the brain hemispheres of Balb/c mice of different age groups: 1) newborn mice, 2) young 2 months old mice, 3) old 1.8 year old mice. We also studied the in vivo effect of recombinant erythropoietin on the expression of Epo-R mRNA in the brain hemispheres of (CBA x C57BL)F1 mice by RT-PCR. We have detected the existence of Epo-R mRNA expression in brain hemispheres of all the groups, but in old mice this expression was significantly higher. We have discovered a decrease in Epo-R mRNA expression in brain hemispheres of (CBA x C57BL)F1 mice 24 h after in vivo administration of recombinant erythropoietin. The Epo-R mRNA expression in the left brain hemispheres of (CBA x C57BL)F1 was considerably higher than in the right one.