The adsorption of the drug 5-Flourouracil (5-FU) on Hydroxyapatite (HAP) particles having different surface conditions is studied at 37 degrees C. Commercial HAP particles, either as received or soaked in citrate solutions are used as drug adsorbents and compared to a third category of HAP particles which consists of granules obtained by spray drying suspensions of HAP nanoparticles previously precipitated in citrate solutions. The adsorbed amount of 5-FU on the HAP particles surface is conditioned by the existing pre-adsorbed citrate ions which contribute to modify the particle surface charge and thus decrease the drug adsorption. However the obtained results also demonstrate that the particle surface charge is not the unique factor to govern the 5-FU adsorption on the HAP particles surface.