The effect of matrix polymer and filler content on the rheological behavior of hydroxyapatite-filled injection molding grade high-density polyethylene (HDPE) has been studied. Studies of the flow curves revealed that the matrix and the composite exhibit three distinct regions in the flow curve, namely, a pseudoplastic region at low to moderate shear rates, a plateau and a second pseudoplastic region at high shear rates. The shear stress corresponding to the plateau (tau(c)) is dependent on both the filler concentration and the melt temperature. Addition of HA in the HDPE matrix increases the value of tau(c) and decreases compressibility of the melt. An increase in temperature also raises the value of tau(c). From the nature of flow curves it is concluded that the matrix polymer largely decides the rheology of the composite.