In this study, to clarify whether the functional capacity of hemopoietic progenitor cells and the micro-environment of aged mice are identical with those of the young, we investigated the changes in the number of hemopoietic progenitor cells and the production of regulatory cytokines from splenic cells as well as changes in the serum levels of cytokine in senescence-accelerated mice (SAM) after administration of 19-nandrolone decanoate (19-ND), a synthetic androgenic anabolic steroid. 19-ND induced an increase in erythroid colony-forming units (CFU-E), erythroid burst-forming units (BFU-E), and granulocytic-macrophage committed progenitor cells (CFU-GM) in bone marrow and spleen; especially remarkable increases were observed in the splenic CFU-E in both young and old mice. Antigen expression analysis of hemopoietic organs revealed that total TER-119+ cells per spleen of young and old mice with androgen treatment rose 2.6- and 3.2-fold over their respective control values. The responsiveness of hemopoietic progenitor cells to androgen did not change with age. Injection of 19-ND into young and old mice markedly enhanced the erythropoietin levels but not IL3 and GM-CSF levels in the serum of both groups. Cytokine production assessed by pokeweed mitogen-stimulated spleen condition medium showed an age-related decline. Androgen treatment could not influence IL-3 and GM-CSF production of spleen. These findings suggest that the spleen of both old and young mice served as the major site of regenerative repopulation of hemopoietic progenitors, especially the late erythroid progenitors in 19-ND-treated mice. The proliferative reserve of erythropoiesis with androgen treatment in aged mice was not reduced more than that in treated-young mice.