ABSTRACT. Intestinal lactase activity is maintained a t high levels in suckling rats during the first 2 wk after birth. When 12-day-old rat pups were either mother fed (MF) or artificially reared (AR) with natural rat milk or several artificial formulas, the small intestines had gained similar weight in all animal groups by 16 days except in AR rats fed a chemically defined formula. In the ileum, villus length was similar in M F and AH rats, but crypt depth was significantly higher in all groups of AR rats. Ileal absorptive cells in both MF and AR rats showed immature characteristics, including supranuclear vacuoles, apical tubular systems, and pinocytotic vesicles. Jejunal lactase specific activity and total intestinal lactase activity were significantly higher in AR rats fed rat milk than M F rats at 16 days. Ileal lactase specific activity was similar in these two animal groups. In contrast, AR rats fed artificial formulas supplemented with either glucose or lactose a s the sole carbohydrate source exhibited significantly lower ileal lactase specific activity and total intestinal lactase activity than M F rats. Intestinal sucrase activity was prematurely elevated in all AH rats, even when fed natural rat milk. Addition of prolactin (3.3 pg/ml) to an artificial formula did not prevent the premature decrease in intestinal lactase specific and total activities in AR rats. We conclude that (I) natural rat milk plays a cardinal role in maintaining lactase activity during the suckling period; (2) the lactose and prolactin content of rat milk are not essential components in maintaining lactase activity in AR rat pups; and (3) the artificial feeding procedure, rather than dietary composition, induces premature elevation of sucrase activity. (Pediatr Res 19: 963-967, 1985) Abbreviations MF, mother fed AR, artificially reared CMF, cow milk formula RCF, Rose carbohydrate-free formula CDF, chemically defined formula G, glucose I,, lactoseIntestinal lactase. which is essential for the hydrolysis and absorption of lactose, is abundant in mammals at birth, remains at high activity before weaning and then generally falls to a low level (I). In thc rat, lactase activity is high for about 2 wk after Rece~ved I-ehrusry 19. 1985: accepted April 30. 1985 birth and declines gradually thereafter to reach adult levels during the 4th wk (2-4). In parallel, intestinal maltase activity, which is low, and sucrase activity, which is absent during the first :?postnatal wk, increase rapidly in the end of the 3rd wk (4). Accumulated evidence indicates that the ontogenetic increase of maltase and sucrase in the rat is modulated by hormones produced by the pituitary-adrenal and pituitary-thyroid systems (5.-7), and the ontogenetic decrease of intestinal lactase activity is regulated in part by the pituitary-thyroid system (8).Previous studies from our laboratory have shown that lactase activity at 24 days postpartum is higher in rats hypophysectomized at 6 days than at 10 days of age (8), suggesting that pituitary hormones have exerted the...