One female was captured with three young attached that were several days old. The young were detached from the female's teats with great difficulty. When these young were returned to the female a few minutes later, after they had been measured, weighed and marked, she attacked them viciously, and within a few seconds had killed all of them by biting their heads. In this instance the dead young were not eaten, although they were temporarily left with the female. Females with young have ample cause for their circumspective demeanor toward adult males, which are especially inclined to eat the newborn. A male engaged in sexual pursuit has been observed to grasp a young dangling behind the female, pull it from her teat, and pausing momentarily, nibble its head off, before continuing to follow the female. Like the genitalia of the post-partum female, the newborn young seem to have an odor that attracts and excites the male. To a lesser degree, adult females also display marked interest in the newborn young of other individuals, which is liable to result in cannibalism. The incidence of cannibalism is affected by the condition, collectively, of the population of voles, and the availability of nutritious food and moisture. In periods of summer drought the grass becomes coarse and fibrous, and its protein content declines. Under such conditions many voles appear to be undernourished, and some are actually emaciated. Dehydration may be an important factor at times when dew is unavailable for drinking and the green vegetation remaining is exceptionally low in moisture content. Voles caught at such times and brought to the laboratory, drank avidly, and gained several grams soon after being offered water or succulence. Cannibalism by adults on newborn young in times of drought may be motivated by the acute need for moisture and nutritious food. In times of drought the birth rate is at low ebb. Adult males have never been observed to display paternal solicitude toward young, but some individuals, kept with females and their litters, did not molest the young and were accepted by the females as members of the family group. Other things being equal, cannibalism involving the young might be expected to be greater at times of high population density. Then, young left in the nest by a female in the course of her foraging would more often encounter adults and partly grown young, both those that lived in the same burrow system and exploring intruders from other areas.