Prostaglandins (PGs) are potent, multifunctional regulators of bone metabolism which have both stimulatory and inhibitory effects. It has also been suggested that there are multiple pathways and receptors for PG responses in skeletal tissues involving different receptors. To investigate the effects of inhibitors of PG synthesis on osteogenesis with various osteogenesis-related molecules, pregnant mice were treated with vehicle, NS-398, a selective cyclooxygenase (COX)-2 inhibitor, or indomethacin from days 10.5 to 14.5 of pregnancy. The numbers and sizes of mice embryos were similar in all three groups, but chondrocytes in the hindlimb anlage of embryos were calcified in the order of vehicle, NS-398, and indomethacin-treated groups. In RT-PCR analysis, mRNAs for COX-1 and COX-2 were inhibited in the indomethacin-treated group, but COX-1 mRNA increased in the NS-398 group. TGFbeta1 mRNA was expressed in all groups and increased substantially in the indomethacin-treated group. Cbfa1 mRNA increased in NS-398 and indomethacin treated groups. There were an increase for osteopontin mRNA in NS-398 group and a decrease for osteocalcin mRNA in NS-398 and indomethacin treated groups. These results indicate that cbfa1 is required for chondrocyte maturation, and COX-2 may be constitutively expressed, just as is COX-1, and they play an important role in endochondral ossification of mouse embryo. Moreover, the present study suggests that TGFbeta1-stimulated PG production is controlled by feedback loop, and PG-stimulated osteopontin production is elicited by the upstream overexpression.