Using the open ventriculocisternal perfusion method, the rate of cerebrospinal fluid (CSF) production was examined in dogs anesthetized with isoflurane (1.4%) and nitrous oxide (66%) in oxygen. In one group (n = 6) the rate of CSF production did not change significantly when the expired concentration was decreased from 1.4% to <0.15%. In a second group (n = 6) maintained at isoflurane 1.4%, the rate of CSF production decreased by ∼8%/h, similar to the time effect previously reported in controls with this model (a decrease of 4–9%/h). These data suggest that isoflurane causes no significant change in the rate of CSF production and that an increase in CSF volume does not occur during prolonged isoflurane anesthesia. In patients at risk due to increased intracranial pressure, isoflurane may be preferred to anesthetics that may increase intracranial volume,e.g., enflurane or ketamine.