Background: Inadequate healing and high anastomosis leak rates at rectal anastomosis may be due to lack of supportive serosal layer and technical difficulty of low anterior resections. Positive effects of sildenafil on wound healing were observed. The aim of this study was to simulate rectal anastomosis as a technical insufficient anastomosis and investigate the effects of sildenafil on anastomosis healing. Methods: Colonic anastomoses were carried out in 64 rats and randomized into four groups, CA + S, complete anastomoses without sildenafil (10 mg/kg for 5 days); CA + S, complete anastomoses with sildenafil; IA + S, incomplete anastomoses without sildenafil; IA + S, incomplete anastomoses with sildenafil. Half of the rats in every group were sacrificed on post-operative day (POD) 3, half of them sacrificed on POD 7. Tissues from the anastomoses were used for functional, histochemical, biochemical investigations. Results: Sildenafil treatment resulted in increased bursting pressures in IA + S on POD 7 (p = 0.010). Collagen maturity was higher in IA + S on POD 3 and POD 7, CA + S on POD 7 (p = 0.010; p = 0.010; p < 0.007). Collagen content was higher in IA + S on POD 7 (p < 0.001). Glutathione, hydroxyproline levels were similar. Malondialdehyde levels were lower in IA + S on POD 3 (p < 0.001). Epithelization score was higher in IA + S on POD 7 (p = 0.007). Inflammation score was higher in CA + S group on POD 3 and POD 7 (p < 0.001; p < 0.001). Neutrophil score was lower in CA + S on POD 3 (p = 0.005). Conclusion: An increase in collagen content, maturity, and epithelization, a decrease in neutrophil infiltration, oxidative stress and better mechanical strength were observed with the administration of sildenafil. (C) 2014 Surgical Associates Ltd. Published by Elsevier Ltd. All rights reserved.