To provide a suitable delivery system for the calcitonin controlled release 80 mg bioabsorbable polyphosphazene matrices were obtained with entrapped 50 or 250 micrograms calcitonin. The in vitro behaviour demonstrated a release burst for about 24 hours, followed by a period of slow release of the peptide lasting for weeks. Matrices containing 250 micrograms calcitonin were implanted under general anaesthesia in osteoporotic female rats, while a group of animals (control group) received unloaded matrices. After thirty days a second batch of matrices was implanted in both groups to prolong the period of treatment until two months. The explanted matrices were histologically evaluated together with the surrounding tissues, and the dosage of the residual calcitonin was also performed. Results demonstrated the good biocompatibility of the system and the complete release of the calcitonin from the matrices 30 days after implantation. The therapeutic effect, after sixty days of treatment was confirmed by the better densitometric values observed in the femoral bone of treated animals than in controls.