The fate of the major connective tissue polysaccharide hyaluronan, as it appears after release from the matrix, was studied in the Atlantic cod by use of subcutaneous administration of hyaluronan conjugated with fluorescein isothiocyanate (FITC) and labelled with 125I. After administration, the ligand was transported to the heart, which contained 58% of the recovered label after 22 h, whereas 36% remained at the injection site. Uptake in other organs was low. Results from intravenous co-injection studies showed that 125I-FITC-hyaluronan and native hyaluronan were in competition for uptake by the same receptor in cod heart. Fluorescence microscopy revealed that FITC-hyaluronan accumulated in spherical structures and discrete vesicles in endocardial endothelial cells lining the muscular trabeculae of both heart chambers. Immunoelectron microscopy showed that, in these cells, the ligand lined the limiting membrane of endosomes and filled the lumen of late endosomes or lysosomes. We conclude that, in the cod, heart endothelial cells are essential for the turnover of hyaluronan. Atrial endothelial cells were also able to ingest 2-microm latex beads, although these were far more effectively phagocytosed by head kidney macrophages. The present results strengthen the notion that the cod endocardium consists of specialized scavenger endothelial cells, resembling sinusoidal endothelial cells of salmonid kidney and mammalian liver. These cells should therefore be regarded as an important part of the cod reticulo-endothelial system.