L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy) phenyl-2-(1H)-quinolone) is a novel, orally active antagonist at the N-methyl-D-aspartate (NMDA) receptor glycine site. As NMDA receptor antagonism is generally associated with anaesthetic effects, we have examined the electroencephalographic alterations produced by doses of L-701,324 that effectively reduce NMDA-evoked responses in vivo. Microdialysis probes incorporating an electrode were implanted in the striatum of rats and perfused with artificial cerebrospinal fluid (ACSF). Under light halothane anaesthesia, 12 consecutive depolarizations were elicited by switching to ACSF containing 200 microM NMDA for 2 or 3 min, every 20 min. NMDA-evoked depolarizations and EEG were recorded with the microdialysis electrode. L-701,324 (5 or 10 mg kg-1 i.v.) or vehicle were administered 5 min after the 3rd NMDA stimulus. L-701,324 dose-dependently inhibited NMDA-evoked depolarizations, with 10 mg kg-1 reducing these responses by 50% for at least 3 h. The average amplitude of the EEG in the window 0.25-6 Hz (low frequencies) and 6-21 Hz (high frequencies) did not change in the control group. At the higher dose of 10 mg kg-1 L-701,324 transiently increased the amplitude of low frequencies by around 20%. In contrast, both 5 and 10 mg kg-1 significantly reduced the high frequencies to around 70% of control, and this action was sustained with the higher dose. Analysis of the relative EEG power spectra confirmed a small, but persistent shift from high to low EEG frequencies. Our results suggest that L-701,324 slightly strengthened halothane anaesthesia at doses inhibiting effectively NMDA receptor function. Accordingly, the resulting anticonvulsant and neuroprotective actions of L-701,324 may not be associated with marked anaesthesia-like side-effects.