The effects of age on the chiasma frequencies, chiasma position and numbers of univalents at MI in males and females of three strains of mouse were examined. Males showed a slight but non significant rise in chiasma frequency in age due to an increase in bivalents with two chiasmata at the expense of single chiasmata bivalents. In contrast, females exhibited a significant decrease in chiasma frequency with age due to the loss of two chiasma bivalents with a corresponding increase in single terminal chiasmata bivalents. In both males and females there was no significant increase in univalents with age in the strains studied. Of interest was the finding of a greater degree of contraction of the MI chromosomes in the oocytes of old relative to young females, a differential contraction that was independent of culture time. This finding is discussed with regard to the "production line theory" and non disjunction at Anaphase in other strains of mice.