173 rats of different ages were used to observe the changes of lactase activity of small intestine during development. Sucrase and alkaline phosphatse activities were also measured. Another 60 rats were used in order to study the effects of lactose

hydrocortisons and insulin. Our results indicate that lactase activity of the small intestion of fetal rat begins to rise from prenatal period. The lartasc activity sustains a very high level both in jejunum and ileum during tue first 3 weeks of postnatal development

then both decline rapidly to very low adult values. On the contrary

sucrase and alkaline phosphatase activities are high throu-ght the whole adult life.Gastric feeding of lactose and injection of hydrocortisone or insulin reduce the rate of declination of lactase activity

but do not prevent its dropping. All of these facts reveal that lartase deficiency of adult small intestine is the normal result of developmental maturation of this enzyme in mammalian rat