Physiological responses include three sorts: reversible within an individual's lifetime, fixed irreversibly at some critical period in life, and genetic. Examples of the first and third but not the second sort have been demonstrated for intestinal nutrient transport. Hence, we searched for critical-period programming of sugar and amino acid transport by mouse small intestine. Mice were maintained on either of two rations from gestation through birth, lactation, and weaning until adulthood: a high-carbohydrate, maintenance-protein ration and a carbohydrate-free, high-protein ration. The two groups of mice were then compared in adulthood while both groups were on the former or the latter ration. Early diet has irreversible effects on gut and body size; because of higher growth rates until weaning mice receiving high-carbohydrate diets achieved and maintained higher weights, longer guts, and heavier proximal guts than the mice receiving carbohydrate free diets. This difference increased with litter size and may have arisen from limitations on nursing mothers' ability to convert dietary protein into milk carbohydrate or fat. Early diet appears to exert some general effects on adult intestinal transport as a result of these differences in body and gut size but does not appear to exert specific irreversible effects on transport of D-glucose, L-proline, L-leucine, L-lysine, or L-aspartate or on passive glucose permeability. Active and passive glucose transport increases reversibly on a high-carbohydrate diet, whereas amino acid transport increases reversibly on a high-protein diet.