The impression that it is very difficult to obtain objectivity and consistency in observations on histological material was confirmed by the reproducibility test to which the method of classification of renal glomeruli was subjected. This test showed a variable and sometimes high degree of inconsistency in individual judgements of glomerular maturity. The inconsistency was most marked when the test was carried out by an observer who had had no previous training in applying the criteria, but it was also increased by fatigue. The average results of groups of counts showed better reproducibility than the results of single counts. It appeared that, as would be expected, better reproducibility was obtained with good histological preparations than with poor ones. It apparently made no significant difference to the results whether the counts were made by the "column" or the "slide" method. It was concluded that the method of classification and counting of renal glomeruli used in the test could give a general picture of glomerular maturáion but was not sufficiently objective to give reliable detailed quantitative information. It appears that maturation of renal glomeruli takes place in three phases with a period of rest preceding the final maturation. The first phase is one of nephrogenesis which in most cases is over by the thirty -sixth week from conception and is not affected by the incident of birth when this occurs prematurely. During the second phase all glomeruli mature gradually until about half are of adult form and the rest are at the stage preceding full maturity. This phase is over in most cases by the end of the first year of post natal life. There then follows the resing phase, which lasts until about the end of the fifth year, in which there is little or no change in the pattern of glomerular maturity. Phase three, that of final maturation during which the remaining immature glomeruli reach adult form, is completed in most cases by the age of nine, though in about a third of this series it lasted until nine to twelve years of age. A study of the changes in the relative glomerular and secretory tubular mass in the renal cortex indicated that, as development proceeds from the thirtieth week from conception to the tenth year of post -natal life there is a marked fall in the proportion of the cortex occupied by glomeruli and a corresponding rise in the proportion occupied by secretory tubules. In the most immature kidneys the ratio of glomerular mass to secretory tubular mass was 14.5%. but by the tenth year it had fallen to 5.4%. This fall in relative glomemlar mass was apparently arrested between the ages of ten months and four years, which corresponds closely with the resting phase of glomerular maturation. It appeared that the changes in the glomerular tuft epithelium used as criteria of glomerular maturity were directly related to glomerular size suggesting that these changes may be a simple result of expansion of the capillary tuft.