Summary In this study, we tested the hypothesis that the plasma arginine vasopressin (AVP) response to osmotic stimulation induced by hypertonic saline infusion is blunted in the early and middle stages of Alzheimer disease (AD). Because animal data support stimulatory cholinergic mediation of AVP osmoregulation at a brain level, the AVP response in AD might provide clinically useful information about the status of brain cholinergic systems. Seventeen AD outpatients and eight normal older subjects underwent both a 90-min hypertonic saline infusion and a 90-min control (normal saline) infusion. Substantial increases in plasma osmolality during hypertonic saline infusion were accompanied by substantial and linear increases in plasma AVP in both groups. However, there were no significant differences in AVP responses between AD and normal older subjects. These results do not support the utility of plasma AVP response to hypertonic saline in the assessment of brain cholinergic status in AD.