Because previous measurements of early postnatal, small intestinal fluid, proteolytic activity in man and animals have been contradictory, we characterized the development of luminal protein digestion using a sensitive assay in which iodinated bovine casein was incubated in vitro at 37°C with luminal fluid flushed from the stomach and small intestine of 12-day-old suckling and 31-day-old weanling rats, followed by measurement of radioactivity in trichloroacetic acid-soluble material. Gastric proteolysis at pH 3.2 in the weanling rat was approximately 50-fold greater than that in the suckling rat. Analysis of stomach fluid acid-soluble casein degradation products of weanling rats by chromatography on G-50 Sephadex in the presence of sodium dodecyl sulfate revealed three peaks of radioactivity comprising 65, 18 and 12% of the total product in order of elution. In the jejunum at neutral pH, the proteolytic capacity of the weanling rat was approximately five times that of the suckling rat; such differences were more pronounced in the ileum, however, in which protein hydrolysis of weanling rats was 30-fold higher than that in the suckling rat. In the suckling rat, small intestine proteolytic activity was greater in the jejunum than in the ileum, but in the weanling rat greater protein hydrolysis was present in the ileum. Gel filtration analysis of reaction products in both intestinal segments and at both ages demonstrated two peaks, which constituted 60–70 and 15–23% of the acid-soluble material in order of elution. These studies demonstrate that luminal proteolysis increases in stomach and small intestine during postnatal development in the rat; moreover, segmental differences in proteolytic activity exist. Presumably, these findings reflect the balance between proteases and protease inhibitors from pancreatic secretions, desquamated intestinal epithelial cells, and diet.

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