Hydroxyapatite (HA) is widely considered as reference material for bone regeneration, due to its high mimicry with natural bone inorganic matrix¹. The biological functions of the natural apatite include the ability to exchange bioactive ions with the physiological environment, contributing to modulate the bone cell metabolism. The processing of stoichiometric hydroxyapatite is affected by its chemical composition since ion substitution affects the crystal size and ordering, powder particle size as well its morphology and surface charge. In turn, upon preparation of water-based slurries, these properties affect the rheological properties. The fabrication of 3D apatitic scaffolds with adequate pore size distribution for applications in load-bearing sites is still a big challenge. The Additive Manufacturing (AM) process called Direct Ink Writing (DIW), based on the extrusion of a concentrated hydroxyapatite slurry, can be particularly adapted to resolve the main limitations associated with conventional shaping, porosity and mechanical properties of ceramic scaffolds. In the present work, apatitic (HA) powders doped and co-doped with bioactive Mg²⁺, Sr²⁺ ions were synthesized by wet methods; we analyzed and compared the rheological properties of stoichiometric and ion-doped HA suspensions on the basis of different powder and dispersant concentration, to identify the best conditions to achieve stable slurries. The stability of HA suspensions was assessed by pH and ζ-potential measurements, as well as viscosity and viscoelasticity tests. We found that, among the various investigated parameters, high absolute zeta potential and low specific surface area of ion-doped hydroxyapatite with respective stoichiometric hydroxyapatite strongly impacts the suspension stability, while the dispersant played key role upon increasing the powder amount². Rheological investigations of ion-doped hydroxyapatite slurries further pave the way to find best condition for extrusion-based 3D scaffolds.