Adult female frogs (Rana perezi) have been used to analyze structural and morphometrical changes which affect lymphoid organs after treatment with a single dose of dexamethasone (DX). Special attention was given to the distinct sensitivity observed during two different periods of the year. Morphological differences were found between adult control frogs sacrificed in November and February. In February, frogs showed degenerated thymic reticular epithelial cells but lesser numbers of cortical pycnotic and mitotic thymocytes than in November. Splenic lymphoid tissue was clearly less well developed in February. No significant variations were found in the morphometrical parameters of jugular bodies whereas the number of lymphocytes of both peripheral blood and bone marrow was significantly higher in February than in November. In contrast, frogs sacrificed in February exhibited less drastic variations as well as a more delayed response to DX-treatment than those of November. The reported differential changes could be attributed to possible distinct levels of circulating sexual and nonsexual steroids during both seasonal periods.