Background: The purpose of the research was to investigate the effects of Tarantula cubensis extract (Theranekron, 1:100 / D2, Richter Pharma, Austria) applied during pre and postoperative period on canine mammary tumours. Methods : A total of 30 bitches between the ages of 5-12 were used in the study. The breed distribution of the bitches was observed as 21 Terriers, two Pekinese, two St. Bernards, one Setter, one Danua, one German Shepherd, one Doberman and one Cocker. Group I consisted 20 bitches with mammary tumours  and subcutaneus injections of Tarantula cubensis extract were performed preoperatively for three times at one week intervals with the dosage of 3 ml per 10 kg. Between the days of 7th and 10th following the third injection, complete unilateral mastectomy was performed and injections were repeated at the 1st and 5th month of postoperative period. Histopathologic results of this group revealed that 10  mammary tumours were malignant and 10 was benign. Group I was compared to Group II consisted of 10 bitches with malignant tumours which were treated by surgical excision alone. Results: In group I, no change was observed with palpation or inspection after the first Tarantula cubensis extract injection. A week later the second injection there was 10% regression in 10 cases. Benign tumours (n=10) were observed to be hard and regressed 50% in the week following the 3rd Tarantula cubensis extract injenction performed preoperatively while malignant ones showed no regression but became hard. Reoccurence on the intact mammary chain was not observed for a year of postoperative period in Group I.  In Group II reoccurence was observed in six bitches and pulmonary metastasis was determined in two. Conclusion: It was determined that Tarantula cubensis extract applications in the bitch resulted in regression and hardness of benign mammary tumours while only hardness was detected in malignant mammary tumours. Postoperative injections can be performed in order to prevent the reoccurence.