To assess the significance of gonadal function in the maintenance of thymic structure, male, sexually mature, 60 days old rats were bilaterally orchidectomized and the volumes of both the thymic compartments, total cellularity of each of them, as well as density (Nv) and mean diameter (D) of thymocytes within the specific thymic regions were stereologically analyzed 3, 7 and 15 days later. Three days after the orchidectomy a decrease in the thymic weight, and a reduction of both the cortical and medullary volumes, were observed. The reduction in cortical volume was followed by a decrease in overall number of the cortical thymocytes, suggesting an enhanced process of thymocyte death. In the same animals, the total number of the medullary thymocytes as well as D remained unchanged. Seven days after orchidectomy both volume of the cortex and total number of the cortical thymocytes reached the control values, while all the changes observed in the medulla of rats 3 days post castration, persisted. In addition, 7 days post castration, the volume of the interlobular connective tissue was increased and this persisted at day 15 post castration. The results indicate that orchidectomy can affect the thymic structure, and possibly the process of intrathymic T cell maturation; the extent of changes depending on the duration of the gonadal deprivation.