As a first step toward assessing the status of brain neuropeptide systems that may be involved in Alzheimer's disease (AD), the cerebrospinal fluid (CSF) concentrations of the neuropeptides arginine vasopressin, somatostatin, oxytocin, and beta-endorphin were measured in patients with AD, normal elderly subjects, and normal young subjects. The plasma arginine vasopressin level was also measured in the three groups. The CSF arginine vasopressin level was significantly lower in patients with AD than in either elderly or young normal subjects, but oxytocin and beta-endorphin levels did not differ between groups. The CSF osmolarity also did not differ between groups. The plasma arginine vasopressin level did not significantly differ between groups, but high plasma arginine vasopressin values were absent in the patients with AD. The CSF somatostatin level was significantly lower in patients with AD than in normal elderly persons, but it did not differ in young normal subjects. These results suggest that central vasopressinergic activity may be decreased in AD and confirm reports of low CSF somatostatin levels in AD.