A single-blind, crossover study was carried out in 9 healthy subjects to investigate the effect of a single dose of 2 tablets of 400 mg pentoxifylline ('Trental' 400) or placebo on cerebral blood flow, cerebral perfusion quotient and mental functions under experimental hypobaric conditions. Subjects were exposed in a low pressure chamber to simulated atmospheric pressures at sea level, 4000 and 7000 metres, and tests were performed 2 hours after intake of the dose, using 99m technetium brain scintigraphy and psychometric tests, to assess acutely induced impairment of cerebral function. After placebo, cerebral blood flow was decreased, especially at the 7000 metre level, whereas there was significantly less decrease after pentoxifylline. The perfusion quotient was normal throughout the pentoxifylline period but was outside the normal range in 8 of the 9 subjects after placebo. Psychometric testing showed better performance by the subjects after drug than after placebo intake. The results suggest that pentoxifylline may protect cerebral blood flow, nutrition and function under impaired supply conditions.