Approximately 92 per cent. of the growth of the fætus of the rat takes place between the fifteenth day and term, i. e. during the last third of pregnancy.Approximately 54 per cent. of the increase in the weight of the mother during pregnancy occurs after the fifteenth day.The gross increase in weight due to the pregnant condition is 71 grams, which is roughly equivalent to 41 per cent. of the body‐weight of the rat.The “permanent” increase in weight during a single pregnancy (no lactation) is considerably in excess of the increase which takes place over the same period in virgin rats.Rats injected with alkaline extract of anterior pituitary gain an excess of 100 per cent. in weight over controls during the first seven days of pregnancy; this increase is retained, and is comparable with that of virgin rats under the same treatment.The author desires to acknowledge the assistance and advice of Drs Wiesner and Robson in the work recorded in this and the preceding paper.