[ ABSTRACT] AIM:To illustrate the effect of methylene blue ( MB) on T-lymphocyte subsets and oxidative stress of the thymus in rats with paraquat ( PQ) poisoning.METHODS: Male SD rats ( n=40) were randomly assigned to 4 groups:normal group, control group, low-dose (2 mg? kg-1? d-1 ) MB group and high-dose (4 mg? kg-1? d-1 ) MB group.After 72 h, the pathological morphological changes of the thymus were observed under microscope with HE staining. The SOD activity and MDA content were measured by colorimetry method.The expression of Bcl-2, Bax, CD4 and CD8 was determined by immunohistochemical method.RESULTS:In MB group, the thymus tissue showed good corticomedul-lary demarcation.MDA content decreased and SOD activity increased, indicating that the ability of antioxidation enhanced. Bcl-2 and Bax expression was down-regulated, Bcl-2/Bax ratio increased, and CD4 positive cells increased.CONCLU-SION:MB protects against oxidative damage induced by PQ, and regulates the distribution of T-lymphocyte subsets in the cortex and medulla, thus relieving the persistent body damage.