1. Rates of fluid endocytosis of rat liver, spleen, hepatocytes and sinusoidal liver cells have been determined, by using 125I-labelled poly(vinylpyrrolidone) as marker. Poly(vinylpyrrolidone) was injected intravenously into rats, and plasma clearance and uptake by liver and spleen were estimated. From these data, rates of fluid endocytosis of 1.2 and 1.8 ml of plasma/g of protein per day were calculated for liver and spleen respectively. Essentially the same results were found in nephrectomized rats. 2. Hepatocytes and sinusoidal cells were separately isolated by the collagenase/Pronase method, and sinusoidal cells were further fractionated by centrifugal elutriation. Hepatocytes, sinusoidal cells, Kupffer cells and endothelial cells showed rates of fluid endocytosis of 0.96, 9.0, 19 and 13 ml of plasma/g of cell protein per day respectively. Total-body X-irradiation did not influence uptake of poly(vinylpyrrolidone) by spleen, indicating that spleen lymphocytes are not significantly involved in fluid endocytosis. 3. For liver a rate constant of exocytosis of 5% per day was found, whereas for spleen no significant loss of accumulated label could be demonstrated during a 21-day period. 4. Distribution of label over a great number of organs and tissues was measured 9 days after the injection. Liver, skin, bone and muscle together contained about 70% of the label present in the carcass; only spleen and lymph nodes contained more label per g fresh weight of tissue than liver.