Bodenheimer and Sulman (1946:255) concluded from their study of _Microtus guentheri_ that in this species, as in "the cat," "the rabbit," "the ferret," and a few other mammals, ovulation is induced by copulation, and that there is no regular vaginal cycle. Hoyte (1955:412) disagreed with these conclusions for other species of _Microtus_, as he trapped individuals of _M. oeconomus_ that had recently ovulated without copulation (at least no sperm were found in the genital tracts). In _M. ochrogaster_ oestrus seems to be controlled largely by the food supply, at least the incidence of perforate females was found to fluctuate irregularly tending to follow the trend of rainfall, and, probably in more direct correlation, the amount of new grass present (see Table 1, and Martin, 1956:383-384). It therefore seems unlikely that in this species ovulation is dependent on copulation. In females that have not yet produced young the teats are minute and well concealed in the fur, so that they are difficult to find, but in lactation they become conspicuous. In early lactation the teats are typically about 1 mm. in diameter and 2.5 mm. in length. As lactation progresses, they become thickened to nearly twice the original diameter. After lactation, as inversion occurs, they shrink to scabrous low prominences, 2 mm. to 3 mm. in diameter, surrounded by bare skin. There are three pairs of mammae, one pair pectoral and the other two abdominal. As mentioned by Jameson (1947:146), the pectoral mammae show little evidence of use in lactating prairie voles. Probably they are not used at all except in females with more than the four young in a litter accommodated by the abdominal mammae. As in various other rodents, the suckling young may cling to the female's teats and may be dragged over the ground as she moves about. When the female forages near the nest, she may drag the young with her instead of leaving them, but she can detach them instantly if she so desires. On many occasions females found in live-traps had young that were several days old clinging to their teats. In some instances young that had their eyes open may have followed the female into the trap and attached afterward. SEASONAL INCIDENCE OF BREEDING In the region of my study the prairie vole breeds the year round, but the rate of breeding changes continually. There is no regularity in the trend of the breeding season from year to year. It is obvious that the species is responsive to environmental changes and is so well attuned that its breeding is speedily initiated or inhibited by changes to favorable or unfavorable weather. The incidence of breeding is highest when temperature is moderate and both water and foods of preferred sorts are plentiful.