Congenic LEW.1W (RT1u) heart grafts in LEW.1A (RT1u) recipient rats are rejected within 15 +/- 6 days. Tolerance (> 100 days) can be induced by pretransplant donor-specific blood transfusions. In both cases, the graft is heavily infiltrated by recipient cells, and class I and class II molecules of the major histocompatibility complex are strongly expressed. Moreover, T lymphocytes extracted from both tolerated and rejected grafts are similarly cytotoxic in vitro against donor cells. However, it cannot be excluded that this cytotoxicity does not operate in vivo. To answer this important question, we have studied the expression of granzyme A and perforin mRNA expression, in situ, by Northern blotting. Our data show that the two corresponding mRNAs accumulate with the same kinetic and at the same level in rejected and tolerated grafts. These results strongly suggest that infiltrating cells are cytotoxic in vivo and that a "cellular rejection" does occur in the "tolerated" grafts. In addition, these findings show that cytotoxic T lymphocytes are not sufficient to impair graft survival and lead to a revaluation of the relevance, in general, terms of granzyme and perforin expression as a correlate of rejection.