A simple implanted device was used to occlude acutely the left middle cerebral artery (MCA) of 32 conscious cats. Groups of 8 cats each were treated with continuous cerebrospinal fluid (CSF) drainage, mannitol (1 gm/kg i.v.), or a combination of continuous CSF drainage and mannitol (1 gm/kg i.v.). Eight cats served as a control group. The neurological status of cats treated with mannitol improved transiently. Perfusion with a mixture of colloidal carbon and buffered paraformaldehyde was carried out 12 hours following MCA occlusion. Gross swelling of cerebral tissue, distribution of colloidal carbon, and breakdown of the blood-brain barrier to fluorescein were similar in the 4 groups. Reduction of mean capillary luminal diameter to 4.5 ± l.Oμ (control 6.5 ± 1.0μ) in the left Sylvian cortex was unaltered by treatment. A significant difference in the distribution of severe neuronal alterations was not demonstrated.