Background: Recently, there has been a focus on the effect of the nonsteroidal anti-inflammatory drugs on the anastomotic leakage rate after colorectal surgery. Methods: An experimental, randomized, placebo-controlled prospective study on 32 male Wistar rats was carried out. We examined the effect of diclofenac 4 mg/kg/day on the cyclooxygenase-2 (COX-2) enzyme in the anastomotic tissue and on the breaking strength of anastomotic and incisional wounds. The operation was performed with colonic resection and hand-sewn anastomosis. After 3 days, the rats were sacrificed and the breaking strength and the COX-2 content of the anastomosis were measured. Results: There was a significantly reduced level of COX-2 in the rats treated with diclofenac (p = 0.001); no significant differences in any of the breaking strength measurements and no significant correlation between COX-2 levels and breaking strength of the anastomotic or incisional wounds could be found (p = 0.073 and p = 0.727). Conclusion: This study for the first time showed that a diclofenac dose of 4 mg/kg/24 h was sufficient to reduce the level of COX-2 enzymes in the anastomotic tissue in rats. This inhibition of the inflammatory response did not lead to reduced breaking strength of either anastomotic or incisional wounds. Whether there is a detrimental effect of COX inhibition on colorectal anastomoses in the clinical setting remains controversial.