We studied changes in the relative proportions of lymphocyte subsets infiltrating RT1-incompatible transplanted rat lungs during acute rejection. Lungs were examined at different times after transplantation with immunohistochemical staining. In the perivascular and peribronchial region, the ratio of helper to cytotoxic-suppressor T cells was reversed on day 4 when cytotoxic-suppressor T cells became most abundant. In the alveolar region, cytotoxic-suppressor T cells were more abundant than helper T cells from day 3. This rapid change in the proportion of T cell subsets has not been reported for other organ transplants. It may explain why lung allografts are rejected more rapidly than other organs.