In our laboratory we have been breeding many harvest mice, Micromys japonicus (Thomas) for these three years. Three litters were born in last autumn and nineteen litters in this spring. We have observed the changes in the external characters from the time of birth up to the time of weaning. The results obtained are summarized as follows:
1) The average litter size is 2.27 in captivity, while it is 5.32 under natural conditions.
2) The average body weight at birth is 1.15±0.09 grams. The sex ratio (_??_/ _??_) is 1.24, but there is no significant difference in statistical point of view.
3) The first body hair is noticed on the fourth day. Ear-holes open on the seventh day. Each pair of incisors of upper and lower jaws erupts on the eighth to the tenth day. This mouse seems to stop nursing on about the fifteenth day.

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General behavior and food habits of a Japanese flying squirrel, Pteromys volans amygdali (Thomas, 1906), were observed in captivity. Among them, the most interesting is the eating method of leaves and it is as follows: the flying squirrel cuts off about the middle of a petiole of Quercus acuta and holding the lamina with the front paws brings it to its mouth. It begins to eat the lamina from the bottom on the right side of the leaf, upwards to the top, then eats on the left side from top to bottom. Among 25.9 leaves cut off at the petiole in a day, 6.1 leaves are partly eaten and 3.6 leaves are left in perfect shape, in an average. When I give a branch of Tsuga diversifolia, it eats only the new leaves at the top.

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