The effects of changes in serum osmolarity on cerebrospinal fluid (CSF) formation were studied in cats. CSF production rates were measured by ventriculocisternal perfusion both before and after intravenous infusion of glucose solutions. Infusion of glucose, hyperosmolar with respect to serum, increased serum osmolarity and caused a decrease in CSF formation rate; conversely, infusion of hypoosmolar solutions lowered serum osmolarity and increased CSF formation. CSF production and serum osmolarity were found to be linearly related. A 1% serum osmolarity change resulted in a 6.7% change in CSF formation. CSF formation increased by 130% with a serum osmolarity of 265 m0sm/1 and was undetectable with serum of 380 m0sm/1.