Quantification and functional characteristics of intact protein uptake, metabolic behavior, and transmission across the intestinal wall were examined using horseradish peroxidase (HRP) transport through adult rabbit jejunum. Transepithelial HRP fluxes were determined with a modified Ussing apparatus. In Ringer solution, no significant difference was found between intact HRP fluxes from mucosa to serosa (JHRPm leads to s) and those from serosa to mucosa (JHRPs leads to m) (3.12 +/- 0.58 and 3.48 +/- 0.45 pmol.h-1.cm-2, respectively). In the presence of 10 mM glucose, slight net secretion was noted. The transport mechanism was shown to be sensitive to metabolic inhibitors, colchicine, and ammonia. Intracellular transfer and catabolism were estimated by measuring transepithelial fluxes of tritiated horseradish peroxidase (J[3H]HRP). Net absorption of 3H equivalent HRP (91 +/- 32 pmol.h-1.cm-2) occurred chiefly in the form of tritiated degraded catabolites of 2-4 kilodaltons. Comparison of the transepithelial fluxes of intact and 3H equivalent HRP made it possible to estimate that 97% of the HRP was degraded while crossing the tissue from mucosa to serosa and 88% while crossing from serosa to mucosa. The saturable absorption observed for JHRPm leads to s became a nonsaturable process for J[3H]HRPm leads to s. These results fit the existence of at least two functional pathways for intestinal protein transport. The main route seems to involve endocytosis, with striking intracellular degradation, possibly during passage through the lysosomal system. The HRP that escapes metabolic degradation is transported by an alternative route requiring the structural and metabolic integrity of the epithelial cells. Although this route only accounts for a small fraction of HRP transport, it may be of immunological importance.