Objective- Study on the effect of heparin in prevention of abdominal adhesions following abdominal surgery in rat. Design- Experimental design. Animals-Twenty male rats with average weight of 248 ± 5 gr. Procedures- Animals were randomly divided into two equal groups of 10 rats (A and B). After induction of anesthesia, a midline abdominal incision was made and intestines were manipulated with a standard method, using a mini Johns Hopkins bulldog clamp. In group A (treatment) 100U/kg of heparin was administered into the peritoneal cavity and in group B (control) the same volume of saline was administered intrapritoenally. The abdominal incision was closed routinely. In group A 40 U/kg heparin and in group B the same volume of normal saline was injected subcutaneously BID. Seven days after surgery, all rats were sacrificed for necropsy. The number and the location of adhesions were evaluated and tissue samples were taken for histopathological examination. Results- Macroscopic inspection revealed much more adhesions in group B compared with group A. In histopathological sections, the granulation tissue in treatment group was mainly “early” and the number of neutrophils and fibrin formation was less than the control group. Conclusion and Clinical Relevance- Heparin treated rats showed less fibrinous adhesion in macroscopic studies and microscopic slides. Regarding to the results of current study, use of heparin can reduce peritoneal adhesion in peritonitis and can be used after major surgeries in rat.