Corticosterone, the glucocorticoid in rodents, is released from adrenal cortex in response to stress. Sometimes, for use in experiments, infants may be separated from their mother mouse. We studied the stress of mother mouse when she separates from infants. As a result, the concentration of corticosterone in the blood of the mother mouse tended to increase, by pulling her infants apart. In the case of using infant mice for animal experiments, it was suggested that it is necessary to consider the maternal stress.