Treatment of rabbits with indomethacin (10 mg/kg per day) 48 h before mating, and with 20 mg per kg at 12 h followed by 8 mg per kg at 48, 72 or 96 h after mating did not affect the rate of egg transport through the oviduct. Indomethacin treatment at the time of implantation interfered with pregnancy and caused degeneration and resorption of embryos. Inhibition of PG synthesis probably does not directly affect egg transport, but PG appears to be required for the retention of implanted embryos.