Dentate gyrus of the rat hippocampal formation was perfused with the potent inhibitor of GABA uptake, nipecotic acid, by means of an implanted dialytrode. Evoked population spikes in dentate gyrus were decreased in amplitude and often abolished during perfusion. However, multiple (2-4) population spikes developed shortly after nipecotic acid withdrawal. This excitability increase, which presented a pattern of repetitive discharge resembling that following blocking of GABAergic transmission was interpreted as electrophysiological evidence that nipecotic acid can act as a false transmitter 'in vivo', as previously postulated from uptake and release 'in vitro' studies.