Recent data suggests that induction of prostaglandin endoperoxide H synthase-2 (PGHS-2) is critical for the anabolic response of lamellar bone elicited by mechanical strain in vivo. The aim of the present study was to localise PGHS-1 and PGHS-2 in rat tibiae following four-point bending in vivo. Right tibiae of 19 adult female rats were subjected to 300 cycles of bending or sham loading at 2.0 Hz with an applied load of 65 N. At 0, 6, and 24 hr postloading, rats were anaesthetised and perfused with Bouin's fixative. Left and right tibiae were dissected, postfixed for 4 hr at 4 degrees C, decalcified in EDTA, and embedded in paraffin. Serial 5 pM sections were stained for PGHS-1 and PGHS-2 using standard immunoperoxidase procedures. For the first time, immunoreactivity for both PGHS-1 and PGHS-2 was localised in bone cells in situ, in the rat tibia. PGHS-1 was distributed widely in all tibiae, while PGHS-2 showed sparse localisation. At the endocortical surfaces (EcS), osteoblasts, lining cells, and osteocytes close to the surface reacted strongly for PGHS-1, as did intracortical osteocytes. At the periosteal surface (PsS), osteoblasts and cells of the osteogenic region were immunopositive. Immediately after loading, the numerical density (n.mm(-2)) of osteocytes labeled with PGHS-1 was significantly greater in loaded tibiae compared to controls. This increase was not seen after sham loading. At 6 and 24 hr postloading, this difference was no longer evident. Staining for PGHS-2 was sparse compared to PGHS-1. Light to moderate reactivity was observed in osteocytes and canaliculae, but the numerical density of labeled cells was significantly less than that for PGHS-1. Moderate staining was seen in lining cells and osteoblasts at the EcS and PsS of some tibiae. Osteoclasts at the PsS reacted strongly for both PGHS-1 and PGHS-2. There was a similar load-related increase in the density of PGHS-2-labeled osteocytes 0 hr postloading. The labeled osteocyte density had decreased at 6 hr, but remained significantly greater in loaded bones. These results show that both forms of PGHS can be localised in bone cells, with PGHS-1 expressed to a greater extent than PGHS-2. The data also suggest that both PGHS-1 and PGHS-2 may play important roles in the early response of bone to mechanical loading in vivo.