Nitric oxide (NO) is an importan! immunoregulation molecule taking part in ceil immunology and it has been reported to be involved in homograft rejection in organ transplantation. In this study, serum levels of 39 Wistar - Albino types rats with skin transplantation were measured and nitric oxide was investigated whether to be a criterion showing acute rejection. Rats were seperated in four groups; I.: control group. II.: autotransplantated group. III.: only homotransplantation was made. IV.: homotransplantation + immunosuppression with cyclosporin. NO levels were measured in scrum samples taken from the rat in 0.-5. and 10. days and no significant difference was observed when the groups were compared to each other. These results suggest that levels of serum NO can not be a reliable criterion for showing acute rejection in experimental skin homotransplantation.