The effect of physostigmine on the loss of consciousness and respiratory depression induced in rabbits by flunitrazepam, 1 mg/kg, was studied to demonstrate whether the restoration of consciousness and respiration rate result from an increase in central cholinergic activity or from an interference by physostigmine with specific binding of flunitrazepam to its receptors. Physostigmine, 0.1–0.4 mg/kg iv, caused a dose-related reversal of consciousness and respiration rate within 15 min of its injection, which lasted 15–30 min rain depending on the dose. This was associated with peak inhibition of acetylcholinesterase (AChE) in the frontal cortex and medulla, at 15 min, ranging from 35–51%. The analeptic effect of physostigmine in flunitrazepam-treated rabbits was prevented by pretreatment with scopolamine, 1 mg/kg. The effective dose range for physostigmine, 3–12 μmol/kg, is close to concentrations of this agent that inhibit activity in solubilized preparations of AChE from rabbit cortex, 1–3 x 10−8M. However, physostigmine, 10−9−10−4M, failed to displace3H flunitrazepam from specific binding sites on membranes prepared from rabbit cerebral cortex. It is concluded that physostigmine antagonizes the somnolence and respiratory depression induced by benzodiazepines by restoring cholinergic transmission to normal levels. The effective dose range of physostigmine is small, and serious side effects from overdose can occur as a result of excess cholinergic activity at neuromuscular synapses.