BACKGROUND: As a test of possible muscarinic up-regulation, the cortisol response to intravenous (i.v.) physostigmine (an anticholinesterase) was measured in 9 elderly volunteers before and after chronic cholinergic blockade with the muscarinic cholinergic antagonist scopolamine. METHODS: Each of the 9 elderly control subjects was given two physostigmine (0.5 mg i.v.) infusions separated by 21 doses of nightly scopolamine (1.2 mg p.o.). No scopolamine was administered the night before infusions, and glycopyrrolate (0.2 mg i.v.) was administered prior to physostigmine, to block its peripheral effects. Vital signs were monitored and blood samples were collected at six time points surrounding the physostigmine infusion (-10, +10, +20, +30, +50, and +70 min). Behavioral measures and cognitive tests were administered prior to and 30 min after the physostigmine. RESULTS: The cortisol response to physostigmine was greater after the second (post-chronic scopolamine) infusion study compared to the first (p < .05) as measured by an area under the curve analysis of all time points. When individual time points were compared, the mean cortisol response was significantly increased after the second physostigmine infusion at the +50- and +70-min time points (p < .05). There were no significant changes in behavioral rating scales, cognitive tests, or vital signs between the two physostigmine infusion study days. CONCLUSIONS: This study demonstrates increased hypothalamic-pituitary-adrenocortical axis responsivity to a central nervous system cholinergic stimulus after chronic muscarinic blockade in 9 elderly control subjects. It also gives further evidence to support previous suggestions of muscarinic plasticity, specifically postsynaptic up-regulation, in the aging brain following exposure to chronic anticholinergic treatment.