Deep hyperventilation (HV)--pCO2 less than 15 mm Hg--was applied in rabbits with normal intracranial pressure (ICP) and in cats with experimental brain oedema developed after epidural balloon compression. In animals with normal ICP there was an increase of CSF and tissue lactate concentration as soon as after 45 min. of HV. The lactate concentration was increasing all the time during HV. The resumption of normoventilation (2 h) after 10 h of HV was sufficient to bring the lactate level down near normality. Electron microscope studies showed only slight ultrastructural changes without destruction, even after 10 h HV. In animals with increased ICP, brain oedema was the main factor raising the lactate level, while the effect of HV was negligible.