As an organ responsible for generation of T-cell repertoire the thymus occupies a central position in establishment of mature immune response. To assess the potential role of the gonadal steroids in development and maintenance of immunological sexual dimorphism, the effects of gonadectomy pre- and postpuberty on the thymocyte profile of male and female rats were examined. Rats aged 30 days or 75 days were gonadectomized; 30 days later the thymic cellularity was estimated and the expression of the cell surface antigens (CD4 and CD8) and the T-cell receptor (TCR) alpha beta was analyzed by flow cytometry. Regardless of age at surgery, the thymus weight and total thymocyte yield were greater in sham-operated males than females; this sexual dimorphism in thymic cellularity persisted after gonadectomy. Sexual dimorphism in the composition of thymocyte subsets was also evident in sham-operated rats, with males expressing a higher percentage of CD4-8- cells, and remained after gonadectomy of adult rats. In male rats, gonadectomy at day 75 increased the percentage of CD4+8- single-positive and TCR alpha beta + cells. In contrast, in females, ovariectomy decreased the percentages of CD4+8- single-positive, CD4-CD8- double-negative, and TCR alpha beta + cells and increased the percentage of CD4+CD8+ double-positive cells. In the immature rats gonadectomy increased the percentages of CD4+CD8- single-positive and TCR alpha beta + thymocytes and decreased the percentages of double-positive and double-negative cells in males, while in the female it increased the percentage of CD4+8- single-positive thymocytes. Gonadectomy at that age abolished the sexual dimorphism in the expression of accessory molecules (i.e., CD4/CD8), but facilitated gender-specific expression of TCR alpha beta. In conclusion, the results suggest that the gonadal steroids are more important for the development than for the maintenance of the sexual dimorphism in the thymocyte composition.