Nitric oxide (NO) is endogenous mediator of numerous physiological processes that range from regulation of cardiovascular function and neurotransmission to antipathogenic and tumoricidal responses. This study was designed to investigate the possible role of NO during erythroid differentiation in K562 erythroleukemia cells. The chronic myelogenous leukemia (K562) cell line call be triggered in culture to differentiate along the erythrocytic pathway, in response to a variety of stimulatory agents. In this study, K562 cells were induced to synthesize hemoglobin by hemin. We investigated NOx (nitrate+nitrite) levels in uninduced (control) and hemin-induced K562 cell lysates during erythroid differentiation. Our results showed that NO levels decreased significantly oil fourth and sixth day both in hemin-induced and control cells; the decrease was, however, more in hemin-induced group than in control group.