Aim To investigate the effects of serum from rats of different ageson endothelial progenitor cells(EPC) ability to differentiate into mature endothelial cells(EC). Methods Mononuclear cells were obtained from young(1 to 2 month-old) and old(19 to 26 month-old) Sprague-Dawley rats bone marrow by Ficoll density gradient centrifugation and cultured with medium DMEM/F12 in culture dishes,48 h later,the suspending cells were translocated to be cultured in other new flasks coated with fibronectin,the secondary attached cells were characterized as adherent cells which were double positive for DiI-acLDL uptake and lectin binding by direct fluorescent staining demonstrated under a laser scanning confocal microscope;serums from young and aged rats were collected and used to culture EPC.The experiments were divided into four groups: aged rat EPC+aged rat serum,aged rat EPC+young rat serum,young rat EPC+aged rat serum and young rat EPC+young rat serum.After incubation with DMEM/F12 containing 20% serum from young or aged rats(no feta1 blood serum was added),in vitro cultured EPC double positive for DiI-ac-LDL uptake and lectin binding were tested by laser scanning confocal microscopy,also were documented by demonstrating the expression of von Willebrand factor(vWF) with immunofluorescence,vascular endothelial growth factor receptor-2(KDR) and endothelial nitric oxide synthase(eNOS) mRNA expression with reverse transcription-polymerase chain reaction(RT-PCR) methods;in vitro vasculogenesis kit was used to test the participation of EPC to tube formation. Results Young rat serum significantly increases percentage of double-positive cells for DiI-ac-LDL and lectin binding in aged rat EPC cultured in EC conditioned-medium whereas aged rat serum obviously decreases such percentage in young rat EPC;young rat serum also markedly promotes EC conditioned-medium to induce vWF and eNOS(p<0.01) and KDR(p<0.05) expression in aged rats EPC;young rat serum obviously increases the ability of aged rats EPC to form tube structure in vitro after incubation with EC conditioned-medium(p<0.05). Conclusions Young serum significantly promotes EC conditioned medium-induced aged rats EPC differentiation into mature EC whereas EC differentiation of EPC from young rats can be partly inhibited by serum from aged rats;there maybe exist age-related changes of systemic factors in young and aged rat serum which modulate rat EPC differentiation.