Rejection remains a major obstacle to long-term success of pulmonary transplantation. Direct delivery of cyclosporine to lung allografts may produce better control of rejection by generating high intragraft concentrations of drug with decreased systemic delivery and toxicity. The efficacy of inhaled cyclosporine in preventing allograft rejection was compared with systemic delivery by intramuscular injections in a rat model of lung transplantation (Brown-Norway to Lewis). Group 1 animals were given no immunosuppression. Group 2 received a single i.m. injection of 25 mg/kg CsA on the day of operation while group 3 received daily doses on postoperative days 0–3. Groups 4–7 received aerosolized CsA daily for seven days. The aerosol generator produced an airborne concentration of CsA of 180 mg/ m3with a mean particle size of 0.7 μ and estimated pulmonary depositions of CsA of 0.98–3.6 mg/kg/day. Animals were killed on POD 7, and the transplanted lungs graded histologically in a blinded fashion. All control animals showed destructive grade 4 changes by POD 7. Animals receiving high-dose aerosolized CsA (groups 6 and 7) showed minimal changes with a mean rejection grade of 1.3. A single i.m. dose of CsA (group 2) failed to prevent rejection; the mean grade was 2.2. Animals given four i.m. doses of CsA had a mean grade of 1.8. Aerosolized CsA provided significantly better control of rejection than did systemic CsA (groups 6 and 7 vs. groups 2 and 3; P<0.0002 and <0.0054, respectively). Local delivery of CsA by aerosol inhalation is effective in limiting acute rejection of the rat lung allograft.