1. Five rats were volume expanded by infusing a volume of blood equivalent to one-third of the estimated blood volume. In six control experiments the same transfusion was given without expanding the blood volume, as an equal volume of blood was simultaneously removed. Sodium excretion increased significantly more after the blood volume was expanded than in the control experiments.
2. Pairs of rats placed on opposite scale pans of a trip balance were cross-circulated by means of arteriovenous bypasses. The blood volume of the rats could be kept constant by keeping the balance in equilibrium.
3. One of each pair of rats received a blood transfusion which either did or did not expand its blood volume. Sodium excretion was measured in the transfused rat and in the recipient rat, the blood volume of which was kept constant.
4. Sodium excretion in the six recipient rats cross-circulated with rats with an expanded blood volume was not significantly different from that in six recipient rats, cross-circulated with rats given a transfusion which did not expand the blood volume.
5. The cross-circulation experiments were repeated, with the difference that the urine of the transfused rat was reinfused. Under these conditions, sodium excretion in 11 recipient rats cross-circulated with blood volume expanded rats was significantly greater than in eight recipient rats cross-circulated with rats given a transfusion which did not expand the blood volume.
6. In cross-circulation experiments in which a blood transfusion was not given urine reinfusion of one rat did not affect sodium excretion of the other rat.
7. It is concluded that the rise in urinary sodium excretion which occurs in an isovolaemic recipient rat cross-circulated with a urine reinfused rat with an expanded blood volume is due to a change in the concentration of a circulating substance.