We examined bone composition and metabolism after hyperbaric oxygenation (HBO) in Wistar rats with 1-hydroxyethylidene-1, 1-bisphosphonate (HEBP)-induced rickets. Twenty rats at 4 wk of age were divided into four groups of five rats each. The HEBP + HBO group received high dose (50 mg.kg-1.day-1) HEBP injections subcutaneously for 7 days and were then exposed to HBO for 7 days. The HEBP group received only high dose HEBP injection for the first 7 days. Control group A received neither HEBP nor HBO. Control group B received no HEBP injection and was exposed to HBO only for the second 7 days. Both bone mineral and hydroxyproline contents significantly increased in rats in the HEBP + HBO group as compared with the HEBP group. Alkaline phosphatase activity of bone, which is a marker of osteoblastic activity and bone formation, was high in the HEBP + HBO and HEBP groups compared with control groups A and B, although there was no difference between the former two groups. On the other hand, tartrate-resistant acid phosphatase activity, which is a marker of bone resorption, was lower in the HEBP + HBO group than in the HEBP group. These findings suggest that HBO suppresses bone resorption in high osteoblastic activity after the cessation of HEBP administration, and this phenomenon increases total bone mass.