In vitro embryo development is strongly influenced by IVM conditions. Increased duration of IVM may cause aging of the oocytes, which has a harmful effect on the embryo development. Oocyte maturation depends upon the synthesis of several proteins that may play important roles in the cytoplasmic maturation. These experiments were conducted to determine the effect of IVM duration(18-h or 24-h) and medium exchange (at 18 h) on embryo development, and to investigate the protein quantities in IVM medium. Korean Native Cow (KNC) ovaries were obtained from a local slaughterhouse, and cumulus-oocyte complexes (COCs) were aspirated from 2- to 8-mm follicles. Groups of 15 COCs were matured in 50-μL drops of TCM-199 supplemented with 10% fetal calf serum (FBS), 1 μg mL^-1 MFSH, 10 μg mL LH and 1 μg mL^-1 Estradiol-17β for 18 h or 24 h. In vitro-matured oocytes were fertilized using frozen-thawed percoll separated spermatozoa (Day 0) in fer-TALP medium for 20 h and cultured in CR1aa medium supplemented with 0.3% BSA (before Day 3) or 10% FBS (After Day 3). All types of cultures were carried out in an incubator at 39°C, 5% CO₂ in air. The total protein quantity in IVM medium at 18 h or 24 h were compared by 2-dimensional gel electrophoresis using a 10–15% polyacrylamide gradient gels. Data from three replicates were analyzed by chi-square test. The proportions of oocytes reaching the blastocyst stage was significantly higher in 18 h IVM group than 24 h IVM group (Table 1). However, there was no difference detected in blastocyst rate between 18 h IVM group and 18 h medium exchange group. Total protein quantity was reduced between 18 h and 24 h in IVM medium. There were 299 protein spots identified in IVM medium;; there was an increase at 10 spots in the IVM medium analyzed at 18 h and a decrease of 20 spots at 24 h. This study suggests that duration of IVM affects subsequent embryo development. The total protein quantity was decreased between 18 h and 24 h in IVM medium. These proteins may be absorbed into the oocytes and reduce development to the blastocyst stage. However, this may be overcome by IVM medium exchange.