Objective To study the effect of pinealectomy and melatonin on interleukin 7(IL-7) expression of rat thymic epithelial cells (TECs) and its physiological significance. Methods Cultured rat TECs were immunostained with anti-pan-cytokeratin; MTT was applied to investigate the growth of cultured rat TECs with melatonin treatment (1×10SUP>-3/SUP>-10SUP>-9/SUP>mol/L), semiquantitative RT-PCR was applied to assess IL-7 mRNA expression. Totally 110 clean_grade male SD rats were divided into five groups: normal control group, sham operation group, pinealectomized group, pinealectomy+75mg/(kgI>&#/I>8226;d) MLT group, pinealectomy +15mg/(kgI>&#/I>8226;d) MLT group, and took the thymus out in the 4th week and the 8th week, Immunohistochemistry and semiquantitative RT-PCR were applied to observe IL-7 expression in rat TECs. Results Melatonin made no difference on rat TECs growth, but IL-7 mRNA expression in TECs was increased by melatonin treatment in a dose dependent manner; Pinealectomy made no difference on the synthesizing of IL-7 by rat TEC, but 15mg/(kgI>&#/I>8226;d) melatonin treatment resulted in significant increasing of IL-7 expression in certain period. Conclusion pinealgland and melatonin may regulate the differentiation and development of thymocyte by IL-7 expres