An incidence of 80% of ureteral dilatation in normal pregnant women has been ascertained by several investigators of late, and also a sluggishness of ureteral action as indicated both by a delay in the excretion of injected dyes and a prolongation of the contraction intervals of ureteral peristalsis. Since no adequate explanation for the occurrence of ureteric dilatation and sluggishness has as yet been offered, a microscopic study of the wall of the ureter during pregnancy has been made, with a view of ascertaining, if possible, some obstructive lesion in its lower portion. Certain definite hyperplastic and hypertrophic changes both in the musculature and connective tissue could be noted, particularly pronounced in the juxtavesical portion of the ureter, where it passes through the parame-trium. This condition could readily give rise to stenosis in the juxtavesical portion of the ureter, since, in addition to the hypertrophic changes in the wall of the ureter, an encircling ring resulting from hypertrophy of the ureteral sheath develops during pregnancy. The loss of tone of the ureteric muscle in pregnant women may be traced to the depressing effect of bile salts both upon the amplitude and the rate of ureteric contractions, as indicated by the results of experiments conducted along those lines and described in this paper. With the fact established that there occurs during pregnancy a steady increase in the amount of bile acids in the serum of pregnant women, the experiments here detailed bear a close resemblance to the clinical observation. The stimulating effect of adrenalin and ephedrin on ureteral peristalsis is remarkable.