To confirm a possible role of sex hormones in governing the seasonal variations affecting the reptilian lymphoid organs, a morphometric analysis was carried out on the thymus and spleen of turtles, Mauremys caspica, intraperitoneally injected with a single dose of testosterone propionate (TP) at the third week of June when physiological levels of testosterone are low. At 4 and 6 weeks, control turtles show an apparent lymphocyte mobilization both from thymus and spleen with a decrease in the percentage of thymic cortex, numbers of cortical lymphocytes, and mitotic index, but increased numbers of medullary lymphocytes. In the thymic cortex of treated turtles, there is a decrease in the same parameters but they occur in the first 2 weeks, whereas the medullary lymphocytes also undergo reduction at 4 and 6 weeks. In addition, the number of reticuloepithelial cells per area unit decreases at 2 weeks in the thymic cortex of treated turtles. These results are discussed from the point of view of a biphasic effect of testosterone on turtle lymphoid organs: In the first weeks, the changes observed could be attributed to the high levels of testosterone; after 4 and 6 weeks, variations are dependent both on long-term effects of testosterone and on those induced by the high values of corticosterone occurring in summer.