A study of the effects of hyperventilation, hypothermia and urea administration on circulation and cerebrospinal fluid pressure in the dog was made using urea in doses of 2 g./kg. and 6 g./kg. Studies were made in 10 dogs at normal body temperatures and after lowering the temperature to 28–30 C. The dose of urea that brought about a significant reduction in cerebrospinal pressure in normothermic dogs lay between 1 g. and 2 g./kg. In a previous study the dose which brought about an equal response in cooled animals lay between 1/3 g. and 1 g./kg. Adverse effects on the cardiovascular indices of ECG, blood pressure or cardiac output were not seen at either normal or lowered body temperature in either study.