Pouched mice ovulate spontaneously and have a 4-day cycle (3-5 days). The variation was caused by prolonged oestrus. The vagina opened at about 34 days of age and the first oestrus was experienced at 44 days of age. Females experienced several sterile cycles before their first conception, which occurred at an age of about 56 days. The gestation period in most cases was 21 days and implantation occurred about 4-5 days after mating. The females were not receptive post partum. Litter size varied from 3 to 13 with a mean of 7.1 in young primiparous females and 7.9 in adult multiparous females, indicating that fecundity did not increase with age or parity in this species. Mortality of young was highest during the first 2 days post partum. The young were not attached to the nipples and were weaned at 25 days of age. Females did not cycle during lactation. After lactation most females exhibited one or two oestrous cycles without mating or became oestrous and mated without conceiving, resulting in a litter interval of about 53 days.