Owing to the possible importance of steroids in the mutual neuroendocrine-immune influences found in lower vertebrates, we study the structural and morphometrical changes induced by a single dose of dexamethasone, a synthetic corticosteroid, on the lymphoid organs of the frog Rana perezi. The dexamethasone treatment produces thymic involution with massive destruction of cortical lymphocytes, intense peripheral lymphopenia with lymphocyte redistribution to the bone marrow from peripheral blood and spleen. The effects on the spleen were less dramatic than in the thymus, but, the proportion of white pulp underwent a significant decrease and the size of splenic lymphoid follicles diminished. Our results demonstrate that Rana perezi is a corticosensitive species although the induced corticosteroid effects were less drastic than those described in other vertebrates, mainly mammals.