The transport of intrathecally administered iodine131-albumin to the circulating plasma was measured in five patients with cerebral atrophy and three with communicating hydrocephalus. The percentage of the administered dose in plasma 24 hours after injection in both groups was lower than reported by others in control patients, but the two groups did not differ significantly. The most likely explanation for these results is diminished fractional turnover of CSF per unit time in patients with CSF volume expansion, regardless of cause. This test appears to have no value in differentiating cerebral atrophy from communicating hydrocephalus.