Comparison of effect of three cyclo-oxygenase (COX) inhibitors, indomethacin, nimesulide and celecoxib, on the following were assessed: preimplantation loss, postimplantation loss and duration of gestation in Wistar rats. Indomethacin (2.5 and 10 mg/kg), nimesulide (10 and 40 mg/kg) and celecoxib (10 and 40 mg/kg) were administered by gavage daily from days 1-7 for preimplantation loss studies and from day 13 to completion of gestation for postimplantation and duration of gestation studies. Number of animals in each group was six. Preimplantation loss was calculated by subtracting number of implantation sites from number of luteal spots and postimplantation loss was calculated by noting the difference between implantation sites and pups delivered. The higher doses of the three drugs were shown to increase significantly the preimplantation loss, while all the doses of three drugs produced a significant increase in postimplantation loss. Number of animals crossing upper limit of 23-day normal gestation period in Wistar rats was increased in the higher doses. At comparable dose levels, there was no significant difference among the three drugs.