Muscimol (8.8 mumol/kg i.v.) and diazepam (7.04 mumol/kg i.p.) decreased the rate of turnover of acetylcholine in midbrain and cortex of rat brain but failed to change acetylcholine turnover in striatum and hippocampus. The similarity in the profile of the action of diazepam and muscimol on acetylcholine turnover in various brain structures adds support to the view that GABA participates in mediating the actions of diazepam. Since the striatum contains an abundance of GABA neurones and intrinsic cholinergic neurones, it is inferred that the metabolism of acetylcholine, and presumably the activity of striatal cholinergic neurones are not regulated by the activation of GABA receptors. Similar considerations apply to the cholinergic pathway projecting from the septum to the hippocampus.