There is limited literature describing the early events that follow subperiosteal suprabony implantation of particulate hydroxyapatite (HA)/collagen composite grafts. This paper investigates, both in vitro and in vivo, the influence of a water-based environment on compressed collagen fiber cylinders and compressed HA/collagen blocks. Following subperiosteal extrabony implantation of an HA/collagen block, the HA particles were not retained within the original dimensions of the preformed dry block. The separation of the particles was related to the expansion of collagen on hydration and to the amount, degree of cross-linking, and degree of compression of the collagen. Keywords: collagen, collagen hydrophilia, composite implants, hydroxyapatite