Background: The aim of the current work was to study the feasibility of functional gene transfer using the gene encoding for transforming growth factor-β1, a known immunosuppressive cytokine, on rat lung allograft function in the setting of acute rejection. Methods: The rat left lung transplant technique was used in all experiments, with Brown Norway donor rats and Fischer recipient rats. After harvest, left lungs were transfected ex vivo with either sense or antisense transforming growth factor-β1 constructs complexed to cationic lipids, then implanted into recipients. On postoperative days 2, 5, and 7, animals were put to death, arterial oxygenation measured, and acute rejection graded histologically. Results: On postoperative day 2, there were no differences in acute rejection or lung function between animals treated with transforming growth factor-β1 and control animals. On postoperative day 5, oxygenation was significantly improved in grafts transfected with the transforming growth factor-β1 sense construct compared with antisense controls (arterial oxygen tension = 411 ± 198 vs 103 ± 85 mm Hg, respectively; P = .002). Acute rejection scores from lung allografts were also significantly improved, corresponding to decreases in both vascular and airway rejection (vascular rejection scores: 2.0 ± 0.5 vs 2.8 ± 0.6; P = .04; airway rejection scores: 1.3 ± 0.7 vs 2.3 ± 0.8, respectively; P = .02). The amelioration of acute rejection was temporary and decreased by postoperative day 7. Conclusions: The feasibility of using gene transfer techniques to introduce novel functional genes in the setting of lung transplantation is demonstrated. In this model of rat lung allograft rejection, gene transfer of transforming growth factor-β1 resulted in temporary but significant improvements in lung allograft function and acute rejection pathology.