The aim of the present study was to compare the effects of hypericin and fluoxetine in the treatment of companion dogs with tail chasing in Ahvaz district. In the present survey, eighteen dogs with tail chasing were assigned into three equal groups for a three-year period. The dogs were randomly classified based on different treatment groups. During 15 weeks, dogs of group A were given 0.05 mg kg(-1) hypericin orally and dogs of group B received 1 mg kg(-1) fluoxetine, orally. The group C was the control group. Changes in signs of tail chasing were weekly reported by the owners or a veterinarian. Treatment periods were assessed in five intervals: weeks 1-3, 4-6, 7-9, 10-12 and weeks 13-15, respectively. Hypericin (group A) was significantly more effective in the treatment of tail chasing compared with fluoxetine (group B), (p = 0.043). Statistical analysis revealed a significant difference in each group between weeks 1-3 (X(2) = 8.8, p = 0.01), 4-6 (X(2) = 9.1, p = 0.01), 7-9 (X(2) = 7.4, p = 0.03), 10-12 (X(2) = 10.4, p = 0.005) and 13-15 (X(2) = 12.5, p = 0.002). Improvement of behavior in the dogs of group A was significant compared with group B, between weeks 10-12 (X(2) = 5.4, p = 0.02) and 13-15 (X(2) = 7.2, p = 0.007). In conclusion, our survey showed that hypericin was more effective than fluoxetine in controlling signs of tail chasing.