1. We tested the hypothesis that hypervolaemia causes an increase in intrasplenic filtration of cell-free fluid out of the vasculature. To this end we developed a preparation in the anaesthetized rat whereby the splenic vein could be non-occlusively cannulated. 2. Haematocrit and plasma protein concentrations were measured in the splenic afferent and efferent blood supplies. 3. In response to volume loading with saline (1% body weight), there was a sustained increase in the arterial-venous differential of haematocrit, i.e. there was a relative increase in the haematocrit of the blood draining from the spleen. There was no such change in plasma protein concentration. By contrast, this degree of volume loading had no effect on the haematocrit of blood passing through the hindquarters of the animal. 4. Following volume expansion, there was no significant difference in the protein concentration of the plasma and the lymph fluid collected from the splenic lymphatic duct. 5. Distension of the superior vena caval-right atrial junction by means of a small inflatable balloon, caused a similar increase in the splenic venous haematocrit, and again, no change in plasma protein concentration. 6. We interpret these results to mean that, in response to expansion of the intravascular space, there is increased intrasplenic filtration of plasma out of the blood and into the lymphatic system.