Apical endocytosis in the absorptive cells of the suckling rat jejunum was examined in vivo using the intraluminal injection of a range of different lectin-horseradish peroxidase (HRP) conjugates. Con A-HRP, PNA-HRP, LCA-HRP and RCA120-HRP bound strongly to components of the glycocalyx on the plasma membrane of the microvilli, apical coated pits and the small tubular pits at the base of the microvilli of the absorptive cells. The lectin-conjugates on the plasma membranes were endocytosed from the coated pits and small tubular pits, and then transported into coated vesicles, vesicles and small tubules. Lectin-HRP conjugates were later found within the intercellular space. In contrast, SBA-HRP and DBA-HRP bound weakly to the plasma membranes. The absorptive cells demonstrated little uptake or transepithelial transport. When WGA-HRP was injected into the intestinal lumen in vivo, the jejunum absorptive cells formed a deep and wide apical invagination at the base of the microvilli. WGA-HRP bound strongly to the components of the glycocalyx on the plasma membranes of the microvilli, the deep and wide apical invaginations, the apical coated pits and the small tubular pits. The WGA-HRP conjugate was also found in the coated vesicles, tubules, early endosomes, late endosomes, multivesicular bodies and lysosomes, and within the intercellular space. The lectin-HRP conjugate on the plasma membranes however was almost entirely transported into the early endosomes, late endosomes, multivesicular bodies and lysosomes. Therefore, lectin-HRP conjugates may bind to the glycocalyx component on the apical membrane domains, thus resulting in different membrane formations of apical endocytosis by adsorption to the apical plasma membrane specific glycoconjugates in the absorptive cells of the suckling rat jejunum. In addition, WGA induces deep and wide invaginations in the dynamic (not static) membrane domain of the apical plasma membrane in the suckling rat jejunum in vivo.