The effect of ventilation (normo-, hypo-, and hyperventilation) on transverse sinus pressure (TSP), central venous pressure (CVP), mean arterial blood pressure (mABP), and heart rate was studied in 15 patients undergoing neurosurgical treatment in the sitting position for tumors of the posterior fossa, and the findings were compared with the influence of positive end expiratory pressure (PEEP) on these parameters. TSP was not influenced significantly by PEEP ranging from 0 to 15 mbar, whereas CVP increased. At the same time, mABP decreased slightly. In contrast, TSP showed characteristic changes with varying ventilation: during normoventilation [end expiratory CO2 pressure (PECO2), 38 mm Hg], TSP was 3 mm Hg and increased to 7 mm Hg on average with hypoventilation (PECO2, 44 mm Hg), whereas hyperventilation (PECO2, 32 mm Hg) caused a reduction in TSP to the atmospheric range. At the same time, CVP remained unchanged, whereas mABP increased with hypoventilation. Presuming that the risk of venous air embolism is closely related to the level of TSP, our results allow the following conclusions. 1) PEEP does not seem to be effective in preventing venous air embolism. 2) Hyperventilation is dangerous in the sitting position, as TSP is reduced to the atmospheric and even subatmospheric range. 3) To prevent air embolism, moderate hypoventilation is recommended during the most critical period of exposing the posterior fossa followed by normoventilation when surgery of the actual lesion has begun.