Correlation of thymic changes with the development of CsA-associated syngeneic graft-versus-host disease (sGVHD) suggested that the development of tolerance depends on the prompt regeneration of the thymus after stopping CsA. Accordingly, we have tested recombinant human growth hormone (rhGH) and recombinant human insulin-like growth factor I (rhIGF-1) to determine if they accelerate reconstitution of the rat thymus after CsA-induced involution. After 14 days of CsA, the thymus has marked medullary involution but normally recovers fully in 6 weeks. In this study, LEW rats were injected with vehicle, rhGH, or rhIGF-1 for 21 days after stopping CsA and were examined. The vehicle-treated rats showed partial recovery with respect to Hassall's corpuscles, class II antigen expression, medullary size, medullary dendritic cells (DC), and T cell maturation. The mature thymocytes were predominantly CD8+ T cells. Both rhGH and rhIGF-1 induced significant thymic enlargement compared with the vehicle-treated rats. They also both significantly enhanced regeneration with respect to Hassall's corpuscles. The mature thymocyte population had significantly greater CD4+ cells. In addition, rhIGF-1 induced a significant improvement in the medullary size and medullary DC. While the medullae of a normal thymus are in intimate contact with cortical class II antigen, after CsA the cortex adjacent to the medulla is primarily class II antigen negative. RhGH significantly increased the class II antigen in the deep cortex while rhIGF-1 demonstrated a trend toward greater expression in this region (P = 0.06). We conclude that rhGH and rhIGF-1 accelerate thymic regeneration post-CsA. Further studies are now indicated to establish the potential for these factors to enhance the development of antigen-specific tolerance.