Mauremys caspica has been used to analyze morphological changes which affect lymphoid organs (thymus and spleen) throughout the year related to annual variations of circulating steroid hormones. Both organs undergo seasonal variations which, however, differentially affect their distinct compartments (cortex and medulla in the thymus; PALS and PELS in the splenic white pulp). The greatest thymic involution occurs in summer, with some recovery at the beginning of autumn and a slow decrease throughout the winter. At the beginning of spring, although the gland increases somewhat in size, the thymic cortex is only slightly developed. Only at the end of spring does the thymus reach a large size, with a well developed cortex and medulla. In the same season, the splenic lymphoid tissue reaches its maximum development, but in the summer it undergoes a dramatic decrease. These results are confronted with the activity of steroid hormones measured by radio-immune assay, confirming possible relationships between the development of lymphoid organs during summer and spring, and levels of circulating corticosterone and testosterone. The decrease in lymphoid tissue during winter could be related to temperature-dependent membrane homeoviscosity.