ATP is known to be released in association with acetylcholine at synapses in the vertebrate peripheral nervous system. Exogenously applied ATP and its derivatives have been shown to reduce the release of acetylcholine, so it has been postulated that ATP has a role in the modulation of transmitter secretion. More recent results have suggested, however, that specific adenosine receptors are responsible for the inhibitory effects of adenosine derivatives on transmitter release, and ATP, if released, must be hydrolysed to adenosine to produce inhibition. The original hypothesis that ATP itself might inhibit acetylcholine secretion would be strengthened if it were found that adenosine is very much less potent than ATP as an inhibitor of ACh secretion. We report here results which show this is the case in sympathetic ganglia.