Two selected high-fertility mouse lines, namely FL1 and FL2, and a non-selected control (Fzt:DU), all derived from the same genetic pool, were analysed as an animal model for polytocous species to elucidate the effects of long-term selection and to identify relevant component traits that may be responsible for fertility performance. The index trait used for breeding selection was largely increased by 104% and 142% in the FL1 and FL2 lines, respectively, resulting in an average litter size of 17.3 pups and 18.7 pups per litter in the FL1 and FL2 lines, respectively, compared with a litter size of 11.0 pups per litter in the control (Fzt:DU). In addition, different component fertility traits were analysed in females of all three lines at different stages of the oestrous cycle and pregnancy. In conclusion: (1) early embryonic development was accelerated in the FL1 and FL2 lines compared with control; (2) plasma progesterone levels were not correlated with fertility performance; (3) a largely increased ovulation number (i.e. number of corpora lutea) was responsible for high prolificacy in both lines; however, (4) the number of ova shed, as well as the rate of loss of ova and pre- and postimplantation conceptuses, was very different in the FL1 and FL2 lines, suggesting that different genetic components may be responsible for the high prolificacy in both high-fertility lines.