The nuclear maturation and developmental competence of immature, oocytes collected from donors at various morphology of corpus luteum (CL) and fertilized in vitro was investigated by comparing the meiotic activity and the yields of embryos. Ovaries were divided and classified into 4 groups as the following criteria : Group 1 ; ovaries showed evidence of recent ovulation (corpus hemorragicum). Group 2 ; apex of CL was red or brown. Vasculization was limited to periphery of CL. Group 3 ; apex of CL was orange or tan. Vasculization was covered over apex of CL. Group 4 ; CL was light yellow to white and firm in texture and the vascular network on the surface of CL had disappeared. Modified TCM 199 was used for maturation in vitro of immature oocytes and development was induced by using TLP-PVA as a basic medium. When oocytes collected from each group of donors had been matured for 4, 14, and 24 hours in vitro, the proportion of oocytes reaching metaphase I and metaphase II were not different among oocytes from 4 group of ovaries. Mature metaphase II stage of oocytes in each group was first observed at 14 hours, whereas completion of maturation of. oocytes in each group was at 24 hours. Luteal morphology of ovaries had little effect on the proportion of embryos reached 2 cells and 8 cell stage. However, the proportion of embryos cleaved to morula and blastocyst stage was significantly higher in the oocytes obtained from group 1 and 3 than in the oocytes from group 2 and 4 (p＜0.05). This data suggest that reproductive status of the donor significantly influence the yield of in vitro embryos.