Abstract: 【Abstract】  Objective   To observe the effect of a mixture of dihydroartemisinin and metronidazole on ultrastructure of Trichomonas vaginalis trophozoites in vitro for exploring trichomonacidal mechanism of the drug mixture.   Methods  The trophozoites were cultivated with liver extract solution medium that contained 2.5×106 parasites/ml. There were dihydroartemisinin 0.5 mg/ml and metronidazole 0.002 mg/ml in the experimental tubes of the drug mixture group. Groups of control （without drug）， dihydroartemisinin （1 mg/ml） and metronidazole （5 mg/ml） were established and performed in the same experimental conditions. The parasites were observed by scanning and transmission electron microscopes after having treated with the drugs at 37 ℃ for 3.5-5 h. Results Under scanning electron microscope，the cell membrane of T. vaginalis treated only with dihydroartemisinin for 3.5 h was damaged，part of pellicle peeled off. Although the surface of the trophozoites treated only with metronidazole for 5 h showed many small bubbles and hollows，the cell membrane looked integral. However，surface of the parasite exposed to the drug mixture for 3.5-4.2 h showed deep folds and cracks，the cell membrane was damaged and even peeled off. When the cell ruptured，the nucleus， axostyle， pelta and hydrogenosomes were exposed，and the cytoplasm spilled out. Transmission electron microscopy showed that the membrane system of the trophozoites treated only with dihydroartemisinin for 3.5 h was damaged considerably. The cytoplasm of damaged parasite spilled out. The cytoplasm of the parasite treated only with metronidazole for 3.5-5 h was damaged seriously. Vacuoles and crevices were visible in the cytoplasm. The cell membrane and the content of the parasites treated with the drug mixture for 3.5-4.5 h were damaged seriously. There were some vacuoles and crevices， dilated endoplasmic reticulum， injured and deformed hydrogenosomes in the cytoplasm. The cell organelles mostly disap-peared. Crevices also existed in the nucleus. The nuclear membrane fractured and even disappeared.  Conclusion The acting targets of dihydroartemisinin and metronidazole to T. vaginalis trophozoite were different，and a combination of the two drugs shows stronger effect in killing the parasites.

Key words: Trichomonas vaginalis；Dihydroartemisinin；Metronidazole；Ultrastructure；Scanning electron；microscopy；Transmisson electron , microscopy