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A model was proposed for describing the observed in vitro release behavior of drugs from porous HAPs. The model consists of three successive stages; during the first stage, the dissolution medium penetrating into the porous HAP, the amount of the drug released is proportional to the square root of release time. During the second stage, after the pores in the HAP are filled out by the dissolution medium, the drug release being proceeded by dissolution into the dissolution medium outside of the HAP, the amount of the drug released is proportional to release time. During the third stage, after the drug concentration is decreased and below the solubility limit of the drug in the dissolution medium, the drug diffusing to the stirred dissolution medium outside of the HAP, the release rate is markedly slowed and the release amount approaches a plateau value.