The murine endolymphatic sac (ELS) can survive for several weeks in tissue culture. The epithelial cells mature in vitro and demonstrate functional activity under these artificial conditions. The goal of the present study was to investigate fluid and particle transport between the ELS cells, the ELS lumen, and the surrounding tissue culture medium. A series of tracers, including anti-mouse IgG-labelled colloidal gold, horseradish peroxidase (HRP), and ruthenium red (RR) were used to elucidate different aspects of ELS transport. Results indicated that both light and dark cells of the ELS took up HRP from both the luminal and the basal side of the epithelial cells, forming coated vesicles. Only luminal uptake occurred when the HRP was injected into the lumen, and only basal uptake was observed when the HRP was placed in the culture media. HRP introduced into the tissue culture media never entered the lumen of the ELS. Anti-mouse IgG labelled with gold showed no uptake at all from either side of the epithelial cells. Gold could be found only in membrane-bound vesicles in the cytoplasm of reticuloendothelial (RES) cells in the ELS lumen and lateral intercellular spaces (LIS) of the sac. This suggests that there are no IgG sites in sac cells and that the circulating RES cells seen in the lumen of the ELS come from the surrounding connective tissue.(ABSTRACT TRUNCATED AT 250 WORDS)