The cerebral effects of alterations in plasma osmolality (Osm) and colloid oncotic pressure (COP) were examined in normocarbic, normothermic, pentobarbital-anesthetized rabbits that had been subjected to cryogenic brain injury. Monitored variables in all animals included mean arterial, right atrial, and intracranial pressures (MAP, CVP, and ICP), electroencephalographic (EEG) recordings, and cerebral blood flow (CBF). When surgical preparation was complete, a left parietal lesion was produced with liquid nitrogen. Group 1 (control, n = 8) animals subsequently received only maintenance fluids [lactated Ringer's solution (LR)]. One hour after injury, 3 other groups of animals underwent 45 minutes of plasmapheresis, carried out by arterial phlebotomy (packed red cells returned), with separated plasma being replaced by one of three fluids given in amounts sufficient to maintain MAP and CVP at baseline values. The three fluids were 1) 6% hetastarch in hypo-osmotic LR [Group 2 (Hypo-Osm), n = 6; COP = 21 mm Hg, Osm = 130 mOsm/kg]; 2) iso-osmotic LR [Group 3 (Hypo-COP), n = 8; COP = 0; Osm = 305]; and 3) 6% hetastarch in iso-osmotic LR [Group 4 (Iso-Osm/COP), n = 8; COP = 21, Osm = 310]. The animals were killed by exsanguination 25 minutes after completion of plasmapheresis. The brain was removed, the hemispheres separated, weighed, and sliced, and the specific gravities (SpGr) of the regional tissue determined. There were no differences in MAP, CVP, regional CBF, or EEG activity among the groups.(ABSTRACT TRUNCATED AT 250 WORDS)