Fifteen patients who underwent surgery for intracranial tumors under general anesthesia were given mannitol (1 g/kg of body weight) over 30 min. Measurements of mannitol concentration, osmolality, and electrolytes were determined in blood and cerebrospinal fluid (CSF) samples collected over an 8-h period. Seven sets of hemodynamic observations and four sets of intracranial pressure (ICP) measurements were also made. Mannitol disappeared from plasma in a biexponential manner. The mean maximal plasma concentration was 5.91 mg/ml at the end of mannitol infusion and decreased to 0.58 mg/ml after 8 h. The plasma half-life for the distribution phase was 0.16 h and for the elimination phase was 2.44 h. The distribution volume was 17.0 L and total plasma clearance 87.4 ml/min. There were marked interindividual variations of mannitol concentration in the CSF. The mean CSF concentration at 8 h was 97.1 micrograms/ml, started to decline earlier in 10 of 12 patients, and never exceeded 12% of that in plasma. Osmolality in serum increased with a maximum at the end of mannitol infusion (from 292 +/- 7 mOsmol/kg to 310 +/- 14 mOsmol/kg; p < 0.01), whereas CSF osmolality was unchanged, thus giving rise to a positive blood-CSF osmotic gradient of at least 10 mOsmol/kg for about 30 min. At the end of the mannitol infusion, cardiac output and pulmonary capillary wedge pressure increased significantly, whereas the hematocrit decreased. ICP was observed in five patients for 45 min, and mannitol induced a mean reduction from 11 +/- 4 to 4 +/- 2 mm Hg (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)