Intravenous injection of polyethylene glycol 400 to rats (1.5 mmole/100 g body weight) increases blood osmolarity, the total water content in the brain hemispheres exhibiting insignificant changes. Cellular dehydration is accompanied by an augmentation of the volume of extracellular fluid in the intact skull cavity. The increased blood content of the brain is pointed to be a 24% increase in the iron level in its tissues. Skull trepanation enhances the dehydrating effect of polyethylene glycol 400 but does not affect the blood content of the brain.