Acute changes in total body haematocrit were produced in anaesthetized dogs by the rapid infusion of packed red cells, dextran solution or hypertonic mannitol. A reduction of haematocrit resulted in a decrease in filtration fraction, and a rise of haematocrit in an increase in filtration fraction. A direct relationship between water reabsorption (relative to filtered load) and packed cell volume (P.C.V.) was also observed.2. It is suggested that the changes in filtration fraction accompanying changes in haematocrit are mediated largely by a passive alteration of efferent arteriolar resistance resulting from modifications of blood viscosity.
