Mucous glycoproteins of rat Brunner's gland were examined ultracytochemically to elucidate the intracellular localization and sites of glycosylation in relation to the functional polarity of the cell organellae using a combination of hydrophilic resin embedment and postembedding staining with labeled lectins. A few cis cisternae were stained with HPA (Helix pomatia agglutinin, specific for terminal alpha-N-acetylgalactosamine) and DBA (Dolichos biflorus aggulutinin, specific for terminal alpha-N-acetylgalactosamine). Trans cisternae as well as cisternae following secretory granules were stained with HPA and RCA-I (Recinus communis agglutinin I, specific for terminal beta-galactose). UEA-I (Ulex europaeus agglutinin I, specific for terminal alpha-L-fucose) seemed to bind to transmost cisternae rather than trans cisternae. HPA bound to the whole cisternae, but to the cis or trans cisternae depending on a cell type. Apical mucous granules were stained with UEA-I, RCA-I, and HPA, but not with DBA. DGA seemed to be a good marker of the cis side of the Golgi apparatus and cell membrane. The glycosylation of mucous glycoprotein in rat Brunner's gland was partly clarified electron microscopically.