The present report deals with the responses to a water excretion test of two groups of subjects: normal nonpregnant persons, both men and women, and normal pregnant women. Similar responses from a few patients having toxemias of pregnancy have been included merely to give a vista of work in progress. The results of this work on toxemia, though suggestive, are regarded in no sense as final, and work is proceeding along this line as patients with toxemia present themselves for study. It was soon seen by comparison of the responses to tests on subjects who were collected as a control group for the study of toxemia that there was a marked discrepancy in volume among pregnant patients who were all clinically normal. Tabulation of these tests by months of pregnancy showed that the volume capacity decreased progressively from the seventh month on. The detailed data will be given later in