Effects on Na appetite of various alterations in blood volume and tonicity were studied in rats. Na appetite was not increased 8 hr. after subcutaneous polyethylene glycol (PG) injection, at which time blood volume was decreased but serum Na was normal, or 1 1/2 hr. after gastric water loading, when serum Na was decreased but blood volume was normal. However, Na appetite was increased 8 hr. after subcutaneous formalin injection, at which time both blood volume and serum Na concentration were decreased, and 24 hr. after formalin or PG treatment, when serum Na concentrations were normal. The results suggest that, unlike thirst, Na appetite is not immediately associated with changes in thevolume and tonicity of the intravascular fluid.