SUMMARY Apart from a tendency to a slight increase in weight at oestrus, no marked cyclic alterations in body weight are evident in the mouse during the oestrous cycle. In pseudopregnancy, however, a definite weight increase (on average 7%) occurs; this weight gain is rapidly lost after removal of the ovaries, but may be maintained after spaying by the administration of progesterone, thus indicating a close similarity to the weight change of pregnancy.