The interplay between suckling, intestinal growth and brush-border membrane functions is critical during the perinatal period. The present study investigates changes in intestinal dimensions, activities of four brush border membrane hydrolases (lactase, sucrase, maltase and aminooligopeptidase) and rates of sugar and amino acid uptake by intact tissues and brush border membrane vesicles during the first 24 h of suckling. Total intestinal weight, mucosal weight and protein content increased 58%, 80% and 126% (P < 0.05) during the first 6 h of suckling; length and surface area did not increase. Total mucosal DNA content was 4.6-fold higher at 24 h after birth, with the rate of increase differing among intestinal regions. Hydrolytic capacities of the entire small intestine increased, more so for homogenates than for brush border membrane vesicles, and more for lactase relative to the other hydrolases studied. Rates of nutrient transport declined, especially for brush border membrane vesicles, for proximal and mid-intestine relative to distal intestine, and for glucose relative to galactose and amino acids. We conclude that 1) changes in brush border membrane digestive functions coincide with rapid intestinal growth, with postnatal patterns varying among hydrolases, transporters and regions; 2) insertion into the brush border membrane, not synthesis, limits the postnatal increase of hydrolase activity; and 3) despite declines in specific activity, hydrolytic and glucose transport capacities of the entire intestine remained stable or increased, and exceeded estimated dietary loads because of intestinal growth.