The purpose of the study is to investigate the effects of extractum cepae (EC), heparin sodium (HS), and allantoin (AA) on postoperative peritoneal adhesion (PPA) prevention. Forty-eight rats were allocated into six groups (n = 8). PPA was performed by Harris method. In group 1, no additional procedure was done. Abdominal cavity was washed with normal saline in group 2, EC in group 3, HS in group 4, AA in group 5, and EC-HS-AA mixture (EHA-m) in group 6. On the 21st postoperative day, PPA was scored with Nair adhesion score and examined for fibrosis histopathologically and hydroxy-proline levels (OHP) biochemically. Macroscopically, PPA was decreased in groups 2, 4, and 6 compared to groups 1, 3, and 5 (p < 0.05) and decreased in group 2 compared to other groups (p < 0.05). Histopathologically, fibrosis was decreased in groups 2, 4, and 6 compared to other groups and decreased in group 2 compared to other groups (p < 0.05). Tissue OHP level was decreased in groups 2, 3, 4, and 5 compared to groups 1 and 6 (p < 0.05). Tissue OHP level was decreased in group 6 compared to group 1, but the difference was not statistically significant. PPA formation decreases with normal saline, HS and EHA-m, whereas increases with EC and AA both macroscopically and microscopically. Although HS and EHA-m seem to have adhesion prevention effect, washing the intestines with saline seems to be more effective.