Abstract
1 Adenosine, adenosine monophosphate and adenosine triphosphate (ATP) depressed the firing rate of neurones in the rat cerebral cortex when applied by microinontophoresis. 2 4-Aminopyridine, also applied iontophoretically blocked the depressant effects of the purines, without affecting responses to gamma-aminobutyric acid (GABA). This blockade was effected against purine depressions of both spontaneous and glutamate-evoked activity, suggesting that the interaction occurred postsynaptically.