Effects of glucocorticosteroids (GCS) on the thymus and peripheral lymphocytes of Nile tilapia. Oreochromis niloticus were examined. Young fish (5-7g) were injected into intraperitonial cavity with various dose of dexamethasone (DEX) or hydrocortisone (HC). Histology of thymus and thymocyte counts in treated fish were compared to normal ones. The results showed that in vivo adminstration of DEX or HC induced weight loss of thymus and reduction of thymocytes, and both of these were dose and time dependent. In vivo treatment of thymocytes with 10 mM DEX or 10 mM HC for 12hrs caused DNA fragmentation. Both drugs could split the DNA into fragments of about 180-200 base paire multiples. There was no change in granulocytes of peripheral blood due to the treatment of DEX or HC with various length of time. In contrast, treatment of DEX or HC for 2-3 days decresed number of peripheral lymphocytes. The results indicate that the thymocytes and circulating lymphocytes would respond to GCS depending on several factors, such as nature of the hormone, dose, and duration of treatment.