Iron dextran, an electron-dense tracer, was given intravenously (100 mg of iron/100 g of body weight) to 8 normal rabbits to study its movement from the plasma to the peritoneal cavity during peritoneal dialysis. The dialysate infused at 75 ml/kg contained 1.5 g/dl of glucose in 4 animals and 4.25 g/dl in the remainder. Peritoneal dialysis was discontinued and the peritoneum was fixed in vivo at various times (20–120 min) after the injection of the iron dextran. Large amounts of tracer were detected in the effluent after draining the peritoneal cavity. Electron microscopic examination of the mesentery showed particles of iron dextran in the endothethial cells of small vessels (capillaries and venules) the interstitium and the mesothelial cells adjacent to vessels irrespective of the dialysate concentration or duration of dialysis. Tracer was not clearly demonstrated in the interendothelial or intermesothelial spaces. In the mesothelial cells, the particles were found exclusively in (small, elongated or large) vesicles, while in the endothelial cells they were both in vesicles and free in the cytoplasm. Our findings suggest that during the 20- to 120-min period after intravenous administration, the intracellular transport of iron dextran, depends on either moving vesicles or the presence of pre-existing tunnels in the mesothelial cells of rabbit mesenter