Ramorelix is a glycosylated gonadoliberin antagonist patented by Hoechst A.-G. as an anticancer agent. In preclinical studies, a single injection of ramorelix microparticles inhibited tumor progression for only 14 days. This short action is due to a different release profile of the ramorelix microparticles and the different specific activities of peptides incorporated. In the preventive experiments, animals were treated 17 days after DMBA induction before tumor development. Treatment with buserelin implants every 56 days or with buserelin microparticles every 28 days and the treatment with ramorelix microparticles every 7 days prevented the development of tumors. Six weeks after the last injection of ramorelix microparticles a strong tumor progression was seen. There was a clear correlation between peptide release and tumor inhibition. The implants and the microparticles were well tolerated, no tissue reaction or side-effects of ramorelix were seen.