The effects of gas composition in the subarachnoid space (injection of air or N(2)O) and in an anesthetic gas mixture (inhalation with or without N(2)O) on cerebrospinal fluid pressure were studied in 22 patients with pneumocisternography for transsphenoidal craniectomy. N(2)O (66%) anesthesia for 10 min increased cerebrospinal fluid pressure by up to 150% in 7 patients who were intrathecally injected with air. Withdrawal of N(2)O from the anesthetic gas mixture for sixty minutes reduced cerebrospinal fluid pressure to the initial pressure. A second N(2)O administration to the anesthetic gas mixture did not elevate cerebrospinal fluid pressure by as much as the first N(2)O administration. In 7 patients receiving subarachnoid air injection, replacing 66% N(2)O with 66% nitrogen prevented the change in cerebrospinal fluid pressure throughout the operation. In 8 patients N(2)O anesthesia and N(2)O intrathecal injection failed to eliminate the rise in cerebrospinal fluid pressure in 8 patients. Withdrawal of N(2)O from the anesthetic gas mixture for 60 min is recommended to prevent an extreme increase in cerebrospinal fluid pressure during pneumocisternography.