Abstract: Objective To investigate the effects of exogenous NO on the gametocyte infectivity and parasitemia during the early stage of infection with plasmo dium yoelii.Methods The level of parasitemia in blood from infected BALB/c mice and oocyst formation in mosquitoes-Anopheles stephensi were observed before and after dealing with NOC5.Results NO donor NOC5 could inhibit the infectivity of gametocytes to mosquitoes,whose effect had close positive relationship with the NO dosage,but it had no remarkable effect on bloodstage malaria parasites.Conclusion Exogenous NO is an effective molecule,which could inhibit the infectivity of plasmodium yoelii gametocytes to mosquitoes.