In the present experiment, the effects of age and sexual experience at pairing on the reproductive performance in Mongolian gerbils (Meriones unguiculatus) were examined. Animals were divided into eleven groups of 10 or more pairs according to age and sexual experience of both animals at pairing. A female was introduced to each breeding cage in which a male was reared for 2-4 days. Only 5 of 121 pairs were separated from each other because of aggressive fighting at pairing. After pairing, they were observed for 10 days, unless they copulated. After copulation, the male was removed from the cage, and the female was observed until birth or for 28 days after removing the partner male. The mean copulation rate of 5 groups of pairs without sexual experience was significantly lower than that of 5 groups of pairs with a virgin female and the group of pairs in which both animals had sexual experience. The pair of the oldest female and male among the 5 groups pairs without sexual experience showed the lowest copulation rate (20%). In contrast, the pairs of a young female and a mature male showed a high copulation rate, while the pairs with sexual experience in both animals exhibited the highest (100%). Five groups of pairs without sexual experience, especially pairs of a young female and a mature male, exhibited a significantly longer interval (4.9 days) between pairing and copulation compared to pairs with sexual experience in both animals (2.9 days). The interval for pairs with sexual experience was further shortened at the second pairing with the same partners (1.8 days). The present study confirmed that pairs of older gerbils without sexual experience in both animals had difficulty in copulation. Moreover, the present study showed that females of pairs in which both animals had a sexual experience, gave birth at a high rate, even after changing the partner that produced a litter.