The mucosal activities of cathepsin B and D were assayed in the small intestine of newborn pigs and of pigs 24 h, 6 days and 4-8 weeks old, respectively. Before sacrificing these animals, the intestinal capacity to internalise and further transmit macromolecules into the blood serum was evaluated by feeding a marker solution containing bovine IgG, bovine serum albumin and fluorescein-isothiocyanate-conjugated dextran 70,000 (FITC-dextran). No significant differences in the activities of cathepsin B or D were observed in the mucosa from the various age groups of pigs. The transmission of the marker macromolecules into the blood was higher in the newborn pigs than in older postclosure animals, all of which only had low or undetectable serum marker concentrations. No apparent difference in the uptake of the markers into the intestinal epithelium could be detected between that of the newborn, preclosure pigs and pigs 24 h old. Both groups showed a similar pattern of mucosal fluorescence, indicating that there had been a high uptake of all markers into the epithelium. In the 6-day-old pigs, epithelial uptake was only visible in the distal small intestine, whereas no uptake at all could be found in the animals 4-8 weeks of age. The results suggest that intestinal closure in the pig, i.e. the dramatic decrease in the transfer of macromolecules from the intestinal epithelium in the blood at about 24 h of age, is not due to a decrease in the endocytotic ability of the enterocytes, nor to a higher degradation rate within these cells.(ABSTRACT TRUNCATED AT 250 WORDS)