We have studied the effects on bone of three structurally dissimilar non-steroidal anti-inflammatory drugs which inhibit prostaglandin cyclo-oxygenase activity (PGH synthase); indomethacin, flurbiprofen, and piroxicam. We used cultures of half calvaria from neonatal or fetal rats to measure effects on PGE2 production, measured by radioimmunoassay. In four day neonatal rat calvaria, indomethacin inhibited PGE2 release into the medium by 80% at 10(-8) M, while flurbiprofen and piroxicam produced similar inhibition at 10(-6) M. However, at 10(-10) M, treatment with all three compounds resulted in an increase in medium PGE2 concentration of 60 to 120%. To assess the mechanism of this effect, bones were labeled with [3H]-arachidonic acid, washed and cultured in the presence or absence of piroxicam. At 10(-6) M, piroxicam inhibited production of cyclo-oxygenase products and arachidonic acid release. However, at 10(-10) M, there was a substantial increase in labeled products, particularly PGE2, despite a further decrease in arachidonic acid release. In 21 day fetal rat cultures, flurbiprofen was found to increase PGE2 release both in control cultures and cultures which had been incubated with cortisol (10(-8) M) to reduce endogenous arachidonic acid release and supplied with exogenous arachidonic acid (10(-5) M) to provide a substrate. These results indicate that three potent inhibitors of PGH synthase can, paradoxically, increase prostaglandin production at low concentrations. The effect does not appear to be due to increased arachidonic acid release, and could be due to increased PGH synthase activity.