Oocytes were obtained from C57BL/6J and ICR female mice ranging in age from six weeks to 15 months. Both strains demonstrated a significant increase in univalents and a significant decrease in chiasma frequencies at metaphase I with age. Strain differences were observed with respect to the specific type of univalent formation and the regression of chiasma frequency on age. Univalent formation was nonrandom and involved small chromosomes predominantly. Although the percentage of oocytes possessing univalents increased significantly with age, the vast majority possessed only one pair of univalents.