Composite hydroxylapatite (HA)/plaster alloplasts were studied for frontal and parietal cranioplasty. The experimental defects were created in 61 anesthetized cats. They were filled with a moist 2:1 mixture of durapatite/plaster or Interpore/plaster containing 0.85% potassium sulfate. After setting, the implant site was sutured. The filled defects were very firm by physical examination and showed little particle scatter by radiography. At the time this report was prepared only 43 of the cat skulls were available for examination. After sacrifice, postmortem visual examination (VE) of the skulls suggested that new bone had formed over some implants. In most implants, however, VE could not determine whether new bone had formed between the HA particles. Probing examination (PE) of the parietal implants with a periodontal explorer gave a better indication; the presence of new bone rather than soft tissue between the HA particles was confirmed by x-ray microanalysis. The ventral surfaces (VS) of the parietal implants showed greater degrees of bone formation by PE; only 12 of 43 parietal implants showed a high degree of bone formation on the dorsal surfaces (DS) by PE whereas 34 of 43 showed this on the VS. The highly significant difference (^3X) in the amount of bone formation on the VS compared with the DS suggests that this osteogenesis, like that observed in cranial trauma, may be stimulated by the dura which acts as a periosteum by supplying blood vessels to the implant.