1. A frog skin was incorporated into the blood circulation of a dog. Each side of the skin was supplied with blood at a constant temperature, flow and pressure.
2. In the experiments in which the blood volume of the dog was expanded with equilibrated blood, there was a fall in the short-circuit current across all eight frog skins. The fall in current began 10 min after the start of the transfusion and reached its lowest value 15–30 min after the end of the transfusion. The dog showed a simultaneous rise in urinary sodium excretion.
3. In the experiments in which the blood volume was not expanded, there was no change in the trend in the short-circuit current in five of the eight skins. There was a fall in current across the other three skins; the pattern of this fall differed from that which occurred when the blood volume was expanded. There was no rise in the rate of urinary sodium excretion in any of the dogs.
4. It is concluded that when a dog's blood volume is expanded the dog alters the concentration of some circulating substance, and that this change causes a fall in the rate of active sodium transport across the frog skin.