Glucocorticoids are known to inhibit the immune response and involution of the thymus is generally accepted as a reliable sign of the immunosuppressive action of glucocorticoids (Buzzetti et al., 1989). Glucocorticoids have been reported to induce programmed cell death (apoptosis) of cortical thymocytes. They can also affect the thymic epithelial reticular cells (Arendt and Wyllie, 1991). On the other hand, the pineal neurohormone melatonin was recognized as an important immuno-enhancing factor in experimental animals (Poon et al., 1992 and Pozo et al., 1997). It was also found that melatonin played an important role in the control of apoptosis via different mechanisms (Sainz et al., 1995).