Acute intracranial compression in anesthetized dogs gave rise after a short latent period to an increase in blood pressure. Shortly after the pressor effect, an increased flow through both vena cavae was observed. Similar pressure and flow patterns were obtained after the injn. of pressor doses of epinephrine or nor-epinephrine. By contrast, Pitressin had a pressor effect without a notable increase in vena cavae flow. These data suggest that acute intracranial compression causes the release into the blood stream of a pressor substance similar in several ways to the pharmaco-dynamic action of epinephrine or nor-epinephrine. The venous return is increased in all these, and this increased flow may be related to the pressor response. Since Pitressin raised the blood pressure but did not significantly increase the venous return, it would appear that it is not similar to the pressor material liberated by intracranial compression.