The prairie vole is non-territorial and somewhat social. Several or many individuals of both sexes and various sizes may use the same system of surface runways and burrows and even the same nest. In general, members of such a group are mutually tolerant. A strange vole may provoke some hostility at first, but may soon be accepted as a member of a new group. Consequently, there are frequent shifts from one home base to another. Sexual relations are probably more or less promiscuous, although a male and female may rest and travel together in a semi-permanent association. In confinement only those males having markedly enlarged scrotal testes showed interest in females that were in oestrus. Post-partum females especially were eagerly pursued by such males. Anoestrus females are imperforate, and a vaginal orifice is present only during an active oestral cycle or in pregnancy. The perforate condition therefore, is a crude index of breeding activity in the population. In adult females the ratio of those that were perforate usually fluctuated between one-fourth and three-fourths of the total. Only in severe summer drought did the numbers decline below 24 per cent. Normally, breeding continues the year around, but it is temporarily inhibited in unusually cold weather or drought. The highest incidence of pregnancy normally is in late spring and early summer. The ratio of juveniles in the population from month to month and year to year is far more stable than the actual population density. Gestation is 21 days or a little less. The mean litter is 3.37 ± .075 young. Three is the most frequent number per litter, with four, two, and five in that order of frequency. Larger and older females have more young per litter, on the average. Average size is greater in those litters having fewer young. At birth, young are between 40 and 50 mm. in length (typically, 47 mm.), and weigh 2.9 ± .05 grams. At an age of nine days the young have their eyes open, and they may be weaned at an age of approximately three weeks. Young suckle chiefly from the four abdominal teats. The pectoral mammae seem to be inadequately developed, with the result that in exceptionally large litters of five, six or seven young, usually no more than four survive. Until weaning the young spend much of their time attached to the female's teats. She may even drag them behind as she forages. Females that have suckling young become much less tolerant of other voles. Attacks on young, and cannibalism, are common. Adult males, especially, are liable to eat the newborn young. The acquisition of cannibalistic habits by individuals, and seasonal lack of adequately nutritious plant foods may result in the killing off of young in such numbers that the population level is held down.