Determinations concerning the enzymes in the fundal and antral area of the gastric mucosa in 118 fetuses, 6 to 40 weeks gestation age, and in 52 newborns, 1 to 28 days old, snowed an enzyme production in the fundal mucosa beginning with the 3rd month of pregnancy and a slow increase until the end of pregnancy; there were only small quantities of enzymes in the antrum. Through ingestion, the stomach of the newborn rapidly gained its full functional maturity. The enzyme quantities in the fundus were 4 times as high on the 2nd day of life. With the use of graduated Ultracella filters and cellophane filters of fixed sizes of pores, used in an apparatus for high pressure filtration (30 atmospheres of pressure), the molecular weight of the pepsinogen molecules was determined to be approximately 40,000. The enzyme activity of the pepsin was tested according to the method of West, Ellis and Scott. The kidneys are capable of excreting pepsinogen. The quantity of the enzyme in the urine of adults is mainly regulated by the hypophysis-adrenal cortex system. In the amniotic fluid pepsinogen first appeared in traces during the 7th to 8th month of pregnancy. Its quantity and concentration clearly increased until the end of pregnancy, and especially during the last month. Since the fetal cortex of the suprarenal gland achieved a tentative matured condition in the 8th lunar month, the synchronism was intelligible. Lowest values were observed with toxemia and Highest values with erythroblastosis. In contrast to the fetal urine, the urine of the newborn showed an enzyme concentration already 7 to 8 times as large during the 1st and 2nd day after birth, whereas the absolute enzyme quantities increased slowly but continuously and were, by the 10th day of life, 10 times larger.