Electrical field stimulation of the rat caudal artery (0.5-ms pulses at 8 Hz for 3 min) results in the release of norepinephrine (quantified by HPLC-electrochemical detection) and adenyl purines including ATP, ADP and AMP (quantified by HPLC-fluorescence detection). The amount of ATP released from the tissue exceeded the amount of norepinephrine. Because postjunctional α<sub>1</sub>-adrenoceptor stimulation with methoxamine also causes release of ATP, both neuronal and extraneuronal sites may contribute to the overflow of ATP. Results with the α<sub>1</sub>-adrenoceptor antagonist prazosin lend support to this notion. Prazosin (10<sup>–6</sup> M) completely blocked the release of ATP by methoxamine but only partially reduced the release of ATP by field stimulation.