We aimed to evaluate the eficiency of serum testosterone suppression as wel as the potential for agonistic stimulation of serum testosterone during chronic treatment with monthly (3.75 mg) depot formulation of domestic substitute of leuprorelin acetate microspheres for patients with metastatic prostate cancer.Methods: A total of 23 patients with metastatic prostate cancer were enroled in the prospective study and received 6 monthly intramuscular depot injections of domestic substitute of leuprorelin acetate microspheres. Their levels and patterns of serum testosterone suppression and the potential for agonistic stimulation of serum testosterone were monitored folowing injection monthly (3.75 mg) depot formula-tion of domestic substitute of leuprorelin acetate microspheres for 24 weeks.Results: Mean testosterone was 431.4 ng/dL, 119.3 ng/dL, 28.2 ng/dL by week 1, 2, 3 and decreased to less than 15.6 ng/dL by week 4 where it remained throughout the treatment period. Median time to suppression of serum testosterone was 20.7 days. No transient minor “escape” from sup-pression occurred in al patients which was defined as a single testosterone value greater than 50 ng/dL once suppression was achieved. Assessment of agonistic stimulation folowing the second depot injection revealed no pattern of stimulation. Conclusion: We concluded that monthly (3.75 mg) depot formulation of domestic substitute of leuprorelin acetate micro-spheres could provide persistent, stable suppression of serum testosterone throughout the dosing intervals, and that the initial depot injection of this formulation also could provide suficient pituitary desensitization to prevent agnostic stimulation of serum testosterone during chronic treatment.