Cerebral blood volume (CBV) and intracranial (ICP) were examined in dogs during 3.5 h anesthesia with isoflurane (1.4% expired) or fentanyl (continuous intravenous infusion), and after decreasing the concentration of isoflurane to <0.15% expired or discontinuing administration of fentanyl. Isoflurane (1.4%) increased CBV 9–11 % for >3 h but increased ICP for only the first 21.7 ± 1.4 min (mean ± SEM). Fentanyl decreased CBV 7–10% for >3 h but decreased ICP for only the first 20.3 ± 2.7 min. Because both halothane or enflurane increase ICP for <3 h in this model, both isoflurane or fentanyl may be preferred to halothane or enflurane for patients at risk for increased ICP.