Aims: The objective of the study was to assess the pharmacodynamic equivalence of LHRH analogue triptorelin 3-month and 28-day SR formulations. Methods: Patients with documented locally advanced or metastatic prostate cancer were randomized to receive one injection of the 3-month formulation (n = 63) or three injections at 28-day intervals of the 28-day formulation (n = 68). Group-chemical castration rates defined as the percentage of patients reaching a testosterone plasma level ≤0.5 ng/ml were compared at D84 (i.e., 3 × 28 days). Testosterone, LH and triptorelin plasma profiles, and change from baseline in plasma PSA were assessed over 3 months (from baseline to D91). Results: Chemical castration rates were 98 and 96% in the 3-month and 28-day formulation groups, respectively, with confidence interval (two-sided 94.2% CI) of [–8.1%; 9.6%]. Median times to reach chemical castration were 18.8 and 18.5 days (p = 0.86, log rank), respectively. Ratios for mean peak plasma levels and AUC<sub>91</sub> of the two formulations for both testosterone and LH fell within the [0.80; 1.25] equivalence interval. Mean PSA decreases from baseline at D91 were 91.0 and 91.7%, respectively (p = 0.73). Conclusion: Treatments with the two triptorelin formulations over 3 months are pharmacologically equivalent.