The purpose of this study was to evaluate the time-dependent effects of steady-state halothane anesthesia on cerebrovascular variables and their relationship to cerebrospinal fluid (CSF)pH. Eight mongrel dogs underwent a 7-h anesthetic, receiving halothane (1.0% end-tidal), O2(50%), and balance N2. Cerebral blood flow (CBF) was measured by injection of radioactively labeled micro-spheres. CSF was sampled from the cisterna magna and cerebral venous blood from the superior sagittal sinus. Measurements were made at 2 h postinduction and hourly for 5 h thereafter. Total CBF at 2 h postinduction was 148 ± 36 ml·100 g−1·min−1and showed a significant decay over the subsequent 5 h to 70 ± 3 ml·100 g−1·min−1. Regional variations were noted, those areas with highest initial flows showing both a greater relative and absolute reduction in flow. Cerebral vascular resistance increased significantly (39%), as did mean arterial pressure (15%). CSFpH values remained constant throughout the experiment. Arterial blood acid-base physiology was also unchanged. Sagittal sinus PCO2increased significantly from 43 ± 4 to 49 ± 3 mmHg while sagittal sinuspH decreased from 7.31 ± 0.01 to 7.37 ± 0.02, consistent with the normalization of CBF. Cerebral metabolic oxygen consumption did not change significantly. The authors conclude that time-dependent changes in cerebrovascular parameters under prolonged steady-state halothane anesthesia are not due to changes in CSFpH and thus brain extracellular acid-base chemistry.