Although previous studies have described the effects of long-term undernutrition on the process of protein digestion, little data exist regarding the impact of short-term food deprivation on digestive processes. To evaluate the effect of short-term food deprivation on luminal proteolytic activity in the rat, we incubated [125I]bovine casein in vitro at 37 degrees C with fluid flushed from the lumen of stomach and small intestine of 12-day-old suckling and 31-day-old weanling rats that were either fed or food deprived for 12 h/100 g body weight (3 h for sucklings, 12 h for weanlings), followed by measurement of radioactivity in trichloroacetic acid soluble material. Assays were performed at pH 3.2 for stomach and 7.4 for small intestine. In the suckling, gastric luminal proteolytic activity was minimal in both fed and food-deprived animals; jejunal and midjejunal proteolysis was not significantly affected by food deprivation, but the hydrolytic capacity of fluid from the ileum was reduced to 20% of that of fed animals. In the weanling, food deprivation increased gastric proteolytic activity two- to threefold but decreased proteolysis in jejunum and midjejunum. Time-course studies revealed that these effects were further accentuated by food deprivation for 24 h/100 g body weight in both age groups. It is concluded that short-term food deprivation may affect luminal proteolytic capacity and that individual segments of the developing rat gastrointestinal tract respond differently to food deprivation. Presumably, direct and indirect effects of digesta may contribute to these differences.