1. The natriuretic response of normally hydrated unanaesthetized dogs to preferential plasma volume expansion was studied, comparing the response to infusion of hyperoncotic dextran in isotonic saline with that to infusion of an identical volume of isotonic saline.
2. Significant increases in urine flow, sodium excretion, sodium filtration and potassium excretion were observed with each type of infusion.
3. The changes with each type of infusion were not significantly different however, even though plasma volume increased significantly following dextran saline infusion.
4. The natriuretic response of normal dogs to preferential plasma volume expansion appears to be commensurate with the saline load infused rather than the induced plasma volume increase.