In female mice, fertility and fecundity decrease progressively with ageing for unknown reasons. The time of day at which ovulation occurred and the time required for all the follicles to ovulate in young (10–14 weeks), middle-aged (9–11 months) and old (13–15 months) female mice were compared under controlled lighting conditions (12 h dark to 12 h light) to determine the relationship between maternal age and reproductive loss. The number of oocytes present in the follicles and the ampullae were counted at intervals of 1 h after mating. In the groups of young and middle-aged mice, the percentage of oocytes ovulated into the ampullae increased gradually and reached almost 100% at 7 h after the midpoint of the dark period. Whereas, in the group of old mice, it took twice as long (15 h) to reach 100%. However, the mean number of total oocytes remained relatively unchanged (young, 14.8; middle-aged, 16.2; old, 13.8). The prolongation in the time required for all the follicles to ovulate in old female mice may therefore be associated with a low fertilization rate and consequently the age-related decrease in number of offspring produced.
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