We investigated the age-related changes in reproductive function in female rats of the Crl:CD(SD) strain. The estrous cycles were monitored from 6 to 42 weeks of age. Other females were mated with males at 6, 8, 10, 15, 19, 23, 27, 31, 35 and 40 weeks of age and caesarean section was performed on day 20 of gestation to examine their fetuses, and then reproductive parameters were calculated and fetal growth was observed. The percentage of rats exhibiting irregular estrous cycles increased from 23 weeks of age. The copulatory and fertility indices decreased from 35 and 27 weeks of age, respectively. Dams at 6 weeks of age showed low numbers of corpora lutea and implantation sites. Although the number of corpora lutea was not affected by advanced maternal age, number of implantation sites decreased from 27 weeks of age. The pre- and post-implantation loss rates increased from 23 and 31 weeks of age, respectively, and the number of live fetuses decreased from 27 weeks of age. Fetal growth retardation was observed in maternal rats older than 31 weeks of age. The external observations on the fetuses revealed umbilical hernia at 35 to 40 weeks of age. These data indicated that maternal aging affected reproductive function and fetal development from 23 and 31 weeks of age, respectively. It was considered that the appropriate age of Crl:CD(SD) female rats for the assessment of reproductive toxicity were 8 to 22 weeks.