Oocytes of various sizes were isolated from ovaries of sexually mature and prepubertal hamsters and cultured ot determine whether all the oocytes were capable of maturing in vitro. It was found that only the oocytes that had attained maximum size (about 80 mum in the vitelline diameter) were capable of undergoing maturation in vitro. Smaller oocytes (less than 80 mum in diameter) failed either to initiate or to complete maturation. The oocytes which were capable of maturing in vitro (competent oocytes) were present in the ovaries of adult females 136 to 138 hr before ovulation. The competent oocytes first appeared in the ovaries of prepubertal females on the 23rd day after birth, about 10 days before puberty.