1. The acid-base equilibrium of the blood as indicated by determinations of the reserve alkali of the blood remains constant in non-pregnant animals at different age periods. This statement does not imply that the acid-base balance of such animals at different age periods is a stable balance.
2. In pregnant animals of the same age periods as the control animals there may develop an instability of this equilibrium which is either associated with the occurrence of a renal injury and which may be looked upon as a retention phenomenon, or arises independently of such an injury. In this latter group of animals the disturbance in the equilibrium increases in frequency and is earlier in its appearance in the gestation period as the age of the organism increases.
3. In old and in senile pregnant animals some physiological mechanism other than that of the kidney becomes unable to stabilize the acid-base equilibrium of the blood with the result that as the pregnancy advances this physiological state of the organism (gestation) becomes pathological.