1. Pig intestinal sucrase and maltase activities increase markedly, and lactase activity decreases, during the second week of post-natal life. Correlations noted between the time course describing these changes and that found previously to describe a decline in the ability of the pig intestine to take up macromolecules suggest that both events are subject to the same type of developmental control. 2. Injection of epidermal growth factor (EGF) into 3-day-old piglets increase sucrase and maltase activities measured 3 days later. These increases, which are not seen when measuring other hydrolase enzymes, are confined to the mid and distal regions of the small intestine. 3. Dexamethasone injected into 3-day-old piglets inhibits lactase and, on occasion, sucrase activities without affecting other intestinal hydrolases. Significant increases in sucrase and maltase activities also occur in distal intestine following injection of EGF plus dexamethasone into 3-day-old pigs. 4. Cytochemical analysis shows EGF effects on sucrase and maltase activities to be exerted in crypt and basal villus enterocytes produced post-natally. Dexamethasone inhibits lactase activity mainly by acting on mid and upper villus enterocytes produced before birth. 5. EGF appears to increase sucrase and maltase activities by extending the time during which young enterocytes continue to accumulate these enzymes in their brush-border membranes. Dexamethasone appears to cause a more fundamental change in the biochemistry of older enterocytes. accompanied by an increasing ability of these cells to transport neutral amino acids through a sodium-dependent mechanism (see James, Smith, Tivey & Wilson, 1987a).