Superfused rat brain cortex slices preincubated with 3H-noradrenaline or 3H-serotonin and superfused rabbit caudate nucleus slices preincubated with 3H-choline were used to examine the effects of gabapentin (an amino acid chemically related to gamma-aminobutyric acid, GABA) on the electrically evoked 3H overflow. Gabapentin inhibited the electrically (3 Hz) evoked 3H overflow from slices preincubated with the 3H-monoamines in a concentration-dependent manner (at 1 mmol/l by 20-30%), but did not affect the evoked overflow from slices preincubated with 3H-choline. The following drugs did not modify the inhibitory effects of gabapentin: bicuculline, RS-baclofen, GABA, phentolamine, metitepin, cocaine, and the inhibitor of serotonin uptake, 6-nitroquipazine. Gabapentin did not modify the inhibitory effect of GABA on the evoked 3H overflow from slices preincubated with 3H-serotonin. In slices preincubated with 3H-noradrenaline the inhibitory effect of gabapentin was still observed when the stimulation frequency was 10 instead of 3 Hz. In conclusion, gabapentin mimics GABAB receptor activation, but it appears to act by a GABA receptor-independent, as yet unidentified mechanism.