Summary: The histamine-content of foetal lungs was found to be negligible. This is in confirmation of earlier work. At birth the histamine-content of the lung is very high, the concentration being greater than that at either 8, 20 or 40 weeks. The total histamine-content of the lung varies little from birth to 8 weeks in spite of increased size of the lung, but after this period the histamine-content increases. The concentration of the lung histamine expressed as a function of the weight of the animal shows a similar distribution to that of concentration expressed as a function of the weight of the lung. The possible significance of variation in the standard deviation and means in the groups studied is discussed.