Background. We have reported previously that F344 rats develop a spontaneous tolerance to WKY lung allografts and show long-term retention of donor-specific skin grafts placed 35 days after lung transplantation. In this study, we investigated the immunologic mechanisms that may be responsible for the prolonged skin graft survival in animals tolerized with lung allografts. Methods. In the rejection group, WKY skin grafts were placed on normal F344 rats, whereas, in the tolerance group, the skin grafts were placed on F344 rats that had received a WKY lung transplant 35 days before skin grafting. Th1 (interleukin [IL]-2 and interferon-γ [IFN-γ]) and Th2 (IL-4 and IL-10) cytokine as well as transforming growth factor-β1 mRNA expression in skin grafts and in draining lymph nodes were determined by reverse transcription- polymerase chain reaction. Macrophage and lymphocyte infiltration in skin grafts and the number of Langerhans cells in epidermal sheets of the grafts were examined by immunohistochemistry. Results. IL-2 and IFN-γ mRNA expression was significantly decreased in both the skin grafts and the draining lymph nodes of the tolerance group, compared to the rejection group, whereas IL-10 and transforming growth factor-β1 mRNA expression was similar in both groups and IL-4 mRNA was rarely detected. Decreased and delayed CD8+, macrophage, and natural killer cell infiltration in the skin grafts from the tolerance group was also detected. Similar reduction in the number of Langerhans cells in the epidermis of the grafts from both groups was seen on day 1 after skin grafting, and thereafter the number remained stable in both groups. Conclusions. Reduced expression of Th1 cytokines and decreased infiltration of CD8+ cells, macrophages, and natural killer cells in the skin grafts may be responsible for prolongation of skin graft survival in the tolerance group.