Rapamycin was examined for its effects on reversal of ongoing rejection of heart, kidney, and pancreas allografts and on suppression of accelerated heart allograft rejection in the rat. A 14-day continuous intravenous infusion of RAPA by an osmotic pump at 0.02, 0.08, and 0.8 mg/kg/day to WFu recipients, starting 4 days postoperation, significantly protected the BUF heart allografts with a mean survival time (MST) +/- 1 SD of 33.2 +/- 19.8 (p < 0.001), 48.2 +/- 14.8 (p < 0.001), and 107.0 +/- 86.3 (p < 0.001) days, respectively, as compared with 7.2 +/- 0.8 days in vehicle-treated controls. Combination of low dose RAPA (0.02 mg/kg or 0.08 mg/kg) and low dose CsA (2 mg/kg) achieved significantly longer cardiac allograft survival than RAPA or CsA alone. RAPA's effect in reversing ongoing rejection of renal and pancreatic allografts was also significant. The BUF kidney and pancreas in WFu recipients treated with a 14-day course of RAPA (0.8 mg/kg/day starting 4 days postoperation) had an MST of 44.7 +/- 15.9 (p < 0.001) and 46.4 +/- 12.5 (p < 0.001), while in vehicle-treated controls, the grafts were rejected within 10 days. RAPA could also suppress accelerated cardiac allograft rejection. Hyperimmunized WFu recipients were treated with two 14-day courses of continuous i.v. RAPA at 0.8 mg/kg/day before and after BUF heart allografting. Significantly longer survival of the grafts (25.5 +/- 3.7 days, p < 0.001) was achieved compared with that of the vehicle-treated controls (3.8 +/- 1.0 days). One-course RAPA treatment before or after heart transplantation was considerably less effective. RAPA was also shown to prevent the increase of serum IgG levels and to inhibit the production of specific cytotoxic Ab in the rat receiving repetitive immunizations. Such effects presumably contribute to the inhibition of the accelerated rejection. The results of this study suggest that RAPA is potentially useful in treatment of ongoing as well as accelerated allograft rejection.