We examined fertilization and early embryonic development in estrus-synchronized gilt with allyl trenboulone (Regumate) to elucidate any adverse effects on normal development of gametes and early embryos. The ovaries, oviducts and uterine were dissect out at appropriate times, and stored at 4℃ before analysis. The oviducts and uterine were flushed to recover the developing embryos and unfertilized eggs. These eggs were stained with Hoechst 33258 to determine developmental stage accurately. The proportion of gilts with unfertilized eggs were highest in the control group, which may be related to weak estrus. However, lower proportions were found in both Regumate and Regumate + PMS + hCG groups. No differences were found in the rate to fertilization and early embryonic development between control and treated groups, thus suggesting that normal development occurred in the estrus-induced groups. Furthermore, gross appearance and histological examination demonstrated that no evidence of harmful effect was found in the treated groups. It was concluded that Regumate may be safely used to induce estrus synchronization with normal fertilization and early development of embryos.