This investigation deals with the histochemical, biochemical and electron microscopical development of the postnatal epithelium in the small intestine of guinea-pigs. Immediately after birth the enterocytes of the whole small intestine are rich in glycogen. Within 48 hours the glycogen in broken down by intralysosomal digestion and glycogenolysis or disappears from the villus epithelium by extrusion of the absorptive cells. The first loss of glycogen occurs in the jejunum; at the latest it leaves the lining epithelium of the ileum so that a proxoim-distal gradient exists. Afterwards for maximal 14 days fat ist taken up from the mother's milk only by the enterocytes of the jejunum without any signs of endocytosis; the bigger part of the fat leaves the cells by exocytosis and enters the intercellular space. Most of the lipid reaches the lymphatics or is absorbed by marcophages; only small amounts are found in the blood capillaries. The number of the enterocytes engaged in the absorption and passage of fat depends on its quantity in the lumen of the small intestine. During the first days of life everywhere in the small intestine the ultrastructure of the enterocytes is characterized by 2 types of mitochondria (large and small ones with different internal structure). Furthermore in the ileum the absorptive cells contain more lysosomes and a more extensive inframicrovillous membrane system than in jejunum. The membrane system consists of aggregated tubules, vacuoles and vesicles; they are situated below the microvilli and sometimes communicate with the lumen of the gut. The big mitochondria are broken down in the lysosomes or appear in the lumen of the small bowel following extrusion of the enterocytes. The lysosomes are involved in autophagic (digestion of glycogen and cell organelles) as well as in autophagic processes (hydrolysis of molecules from the mother's milk and foreign food). These substances are probably incorporated by means of the inframicrovillous membrane system. With respect to the microvillous hydrolases (lactase, alpha-D-glucosidases, peptidases, alkaline phosphatase) histochemical and biochemical assays were carried out with the same artificial substrate. The results depend on the method employed and the enzyme investigated. Using histochemical techniques and indolyl, naphthly, naphthol AS or naphthylamine derivatives as substrates the activity of peptidases and alkaline phosphatase correspond to that in adult guinea-pigs already at the time of birth; alpha-D-glucosidases (glucoamylase, saccharaseisomaltase) become mature at the end of the first, and the development of lactase is finished after the second week of life. For the biochemical investigations (fluorometric measurements of naphthol and naphthylamine) of microvillous enzymes with naphthyl and naphthylamine substrates a new technique of homogenisation using freeze-dried cryostate sections is successfully employed...