The female Chinese (C.) hamster (Cricetulus griseus) exhibits a progressive decline in litter size after the third litter. The mean number of litters produced was 4.9 +/- .5 for 25 C. hamsters, and the mean age of the C. hamsters at the time of delivering the last litter was 16.4 +/- .5 months. Most aged C. hamsters (17 to 20 months old) retained a relatively normal estrous cycle, although fewer ovulated, either spontaneously or after mating, when compared with younger hamsters (5 to 6 months old). Progesterone injections given to aged (16 to 21 months old), otherwise nonproducing, C. hamsters after mating permitted 61% to become pregnant once again and 34% to deliver viable young. Early infant mortality in these litters was greatest, however, and growth of the young was retarded in comparison with earlier litters. It appears as if age-related changes in the ovary and uterus are primarily responsible for reproductive senescence in the female C. hamster.