A microdialysis technique was used to sample acetylcholine (ACh) from the cerebral cortex of conscious rats. We thus investigated the effects of systemically administered cholinesterase inhibitors (ChEI) such as physostigmine (300 micrograms/kg), heptylphysostigmine (5 mg/kg) and tetrahydroaminoacridine (tacrine, 5 mg/kg) on extracellular ACh levels. Baseline quantities of extracellular ACh could be detected, even in the absence of ChEI. Acetylcholine levels increased to 1100% over baseline within 30 min of physostigmine administration and returned to control levels after 1.25 hr. Heptylphysostigmine elicited a maximal increase of 1000% within 1.5 hr, and the effect persisted up to 9.5 hr. A 500% increase was observed 1.5 hr after tacrine administration, and ACh returned to control levels after 4 hr. Although the ACh effects observed in this study correlated with previously determined levels of acetylcholinesterase (AChE) inhibition, we conclude that measures of cortical AChE activity alone are not sufficient to predict extracellular ACh levels following systemic ChEI administration.