The growth and acquisition of meiotic competence of oocytes from fresh and frozen-thawed primary follicles collected from 10-day-old mice was compared during culture in collagen gels for 12 days. The oocytes contained in primary follicles have a mean diameter of about 48 mm and do not resume meiosis without further growth and development. During the 12-day culture period the mean diameter of the oocytes increased to over 60 mm. The oocytes were capable of resuming meiosis when isolated from the gel and cultured in the absence of follicular cells in a manner similar to that observed in vivo. Freezing and thawing did not affect oocyte growth or the ability to resume meiosis; this demonstrates the possibility of storing large numbers of female gametes for subsequent development.