Studies of small mammals, at Ithaca, N. Y., indicate that there is a pronounced reduction of the population from late fall to the following spring. This reduction may amount to from 40 to 80% of the total population. The probable reason for this reduction is a distinct turnover of the population each winter, the adults of many spp. dying of senility or some other cause after they have passed through a single breeding season. Thus the majority of shrews, deer mice, jumping mice, red-backed mice and spp. of similar size die when they have attained an age of from 15 to 22 months. The evidence for this reasoning is presented.