Acute, bilateral pneumothorax (PT) was produced in 14 newborn piglets. The clinical status of the operated and 14 control animals was monitored by measuring the arterial blood gases, acid-base balance and mean arterial blood pressure. Different brain regions were processed for electron microscopy and albumin immunohistochemistry; water and electrolyte contents were also determined at the end stage of experimental intervention. Electron microscopy showed more intense pinocytotic activity in the endothelium of brain capillaries from PT animals evaluated by morphometry. Statistically significant (p less than 0.01) differences were found in the distribution of pinocytotic vesicles in different brain areas of PT animals. The blood-brain barrier seemed to be impermeable to albumin in all brain regions both in the controls and in the PT group. Parallel with the changes observed in pinocytosis, the water and sodium contents were also increased in the PT group in the parietal cortex (water content 85.18 +/- SD 0.81% vs. 84.10 +/- SD 0.52%, p less than 0.01; sodium content in wet brain tissue 70.94 +/- SD 8.44 mmol/kg vs. 65.09 +/- SD 4.43 mmol/kg, p less than 0.05, in dry brain tissue 481.70 +/- 75.70 mmol/kg vs. 410.15 +/- SD 35.45 mmol/kg, p less than 0.05) and in the cerebellum (water content 83.95 +/- SD 1.08% vs. 83.02 +/- SD 0.89%, p less than 0.05; sodium content in wet brain tissue 60.67 +/- SD 3.16 mmol/kg vs. 55.90 +/- 6.26 mmol/kg, p less than 0.01). However, in other brain regions--especially in the water-shed area--there was no correlation between the changes of pinocytosis and water-electrolyte contents of the tissues. It is suggested that the type of edema developing in this severe cardiovascular/hypoxic collapse is cytotoxic of origin and this fact should be more seriously taken into account in the treatment of the disease.