Pregnant female mice (Mus musculus) housed in the same cage, will, upon parturition, combine their litters into a common nest. Young raised communally in this manner grow faster than young raised by single mothers. Since communal nursing occurs consistently in the laboratory, we analysed several aspects of the behavior. I. In the first group of experiments, we determined which factors were responsible for promoting increased growth rates in communally-nursed pups. Groups containing one mother and 14 young (1/14), one mother with seven young (1/7) and two mothers with 14 young (2/14) were compared. Communal young nursed more than young raised by single mothers. At the same time, each communal female nursed less than a solitary female with a litter of equivalent size. Mammary gland development in communal females was greater than in single females nursing 7 young. The interaction between these factors accounts for increased growth rates in communal pups. 2. The reactions of mice raised in a communal nest and mice raised by single mothers were measured in the open field test and in the emergence test. There was no effect of fostering on "exploration" as measured by activity in the open field. Defecation rates, reflecting "emotionality" were low in males raised in a litter of 14 young, regardless of the number of mothers. Mice of both sexes raised in 1/14 litter groupings were the least "timid" as defined by the emergence test. It thus appeared that communal nursing had little influence on adult emotional behavior. 3. The factors influencing females to combine their young into communal nests were determined. Females and their young, separated from similar groups by a barrier consistently combined their young into a common nest when the barrier was lifted. Maternal experience, age of the young or the presence of nesting materials made little difference in their ability to perform this behavior. Aggressive behavior on the part of the mother with infant litters was directed at the alien young when there was a large age discrepancy between the two litters of pups. The significance of these results is discussed.