OBJECTIVE:To elucidate the function of cytosolic malate dehydrogenase (Mor2) in oocyte maturation and embryo development using RNA interference (RNAi). DESIGN:Experimental animal study. SETTING:Research unit of university. ANIMAL(S):Female 4-week-old (C57/BL6) mice. INTERVENTION(S):Isolation of immature germinal vesicle (GV) oocytes or fertilized pronucleus (PN) embryos, microinjection of Mor2 double-stranded RNA (dsRNA), and reverse transcription and polymerase chain reaction (RT-PCR) analysis to investigate Mor2-specific messenger RNA (mRNA) knockdown. MAIN OUTCOME MEASURE(S):Relative changes in mRNA levels after microinjection of Mor2 dsRNA and in rates of oocyte maturation and preimplantation embryo development. RESULT(S):Mor2 mRNA mostly was knocked down in germinal vesicle- and metaphase I (MI)-arrested oocytes, compared with metaphase II (MII)-developed oocytes, after microinjection of Mor2 dsRNA and in vitro culture for 16 hours. In vitro oocyte maturation was significantly decreased (34%), compared with noninjected (73.4%) and buffer-injected (67.5%) control groups. The rate of blastocyst development (48.1%) was lower in the Mor2 dsRNA-injected group than in buffer-injected control (88.2%). CONCLUSION(S):In the present study, the function of Mor2 was analyzed with the aid of RNAi. On the basis of the data obtained, we propose that Mor2 is an essential factor for oocyte maturation and embryo development in mouse.