1. Sodium, potassium and para-amino hippurate (PAH) transport was studied in renal tubule fragments incubated in vitro in plasma taken from dogs before and after their blood volume was expanded with blood with which the dog was in equilibrium. The individuals who carried out the cell transport studies did not know the identity of the plasma samples.
2. Tubule fragments were less able to maintain a gradient of sodium and potassium, or to accumulate PAH when incubated in plasma obtained after blood volume expansion than when they were incubated in control plasma.
3. It is concluded that expansion of the blood volume causes the dog to change the concentration of some unknown circulating substance which influences sodium, potassium and PAH transport in separated tubule fragments.
4. It is proposed that this substance is partly responsible for the rise in urinary excretion of sodium that accompanies blood volume expansion in the intact animal.