Pial-arachnoidal microvessels (40-210 micron) were studied by fluorescent microscopy in anaesthetized, immobilized and ventilated newborn piglets in the course of bilateral experimental pneumothorax (BEP; n = 10) using the open cranial window technique. Na+-fluorescein and fluorescein isothiocyanate (FITC)-dextran (mol.wt. 40,000 and 70,000 Da) administered i.v. served as blood-brain barrier (BBB) indicators. After gradual exhaustion of compensatory mechanisms a critical phase, characterized by severe acidosis, bradycardia, arterial hypotension following hypertension and arterial hypoxaemia ensued, with vasoconstriction following vasodilation. Moreover, progressive circulation disturbances, sludging and microthrombi formation occurred in small venules. Concomitantly, diffuse BBB opening for Na+-fluorescein ensued in all piglets with BEP as shown by extended fluorescence in the brain tissue around the small venules (less than 80 micron); never observed for FITC-dextran and in the control animals (n = 4) without BEP. In the acute phase of pneumothorax a selective opening of the BBB should be considered.