Abstract: Objective To study the relationship between the change of T cells subsets and the mechanism of thermotolerance formation during hyperthermia.Methods The mice were exposed to hyperthermia by hot air(41℃,relative humidity 70%)in a thermal chamber for 1h each day,and were killed after hyperthermia for 4h at every other day till the nineth day.After weighed,thymuses and spleens were collected as single cells.The T cell subsets were assayed by flow cytometry.Results During hyperthermia,the weight of thymus decreased from the third day,and the weight of spleen decreased at the third and fifth day.The CD4⁺ CD8⁺ thymocytes decreased at the first and third day,but increased from the fifth day.The CD4⁺ /CD8⁺ratio in thymocytes increased in the first day,decreased from the third day,but increased again at the ninth day.The CD4⁺ /CD8⁺ ratio in splenic cells also increased at the first day,but decreased to the lowest at the third day.Conclusion Hyperthermia damaged thymocytes and spleen cells of mice.The weight of thymus decreased was mainly because of the decrease of CD4⁺ CD8⁺ thymocytes.The rates of thymocytes and splenic T cells recovering themselves during hyperthermia may suggests the formation of thermotolerance.