The thymus of the adult frog Rana temporaria is generally small each winter and grows through the spring to reach a large size each summer. The summer thymus has a cortex full of small thymocytes and a medulla in the centre, whereas the winter atrophy is manifested by a loss of distinction between cortex and medulla, an abundance of cells filled with secretory granules, and the formation of intercellular cysts. These seasonal changes are superimposed on age changes. The thymus grows rapidly in froglets. The differences in weight and cell number between winter and summer organs are strongest in middle-aged animals (3-6 years old) and decrease in old specimens. The thymus slowly involutes with age, this being connected with increasing winter atrophy, leading to the formation of huge cysts that fill almost the whole organ in the oldest individuals. In senescent frogs (around 10 years old) seasonal differences still concern corticomedullary division but without pronounced fluctuations in thymic size. The skeletochronological technique applied here for age estimation underestimated rather than overestimated the real age of old animals.