1 Diadenosine polyphosphates, Ap4A and Ap5A, as well as ATP, c,fl-MeATP and ADP-P-S, were able to elicit variable intrasynaptosomal Ca21 increases in rat midbrain synaptic terminals. The origin of the Ca2+ increment was the extrasynaptosomal space since the elimination of extracellular Ca2+ abolished the effect of all the agonists. 2 The P2-purinoceptor antagonist, suramin, did not affect the Ca2+-increase evoked by diadenosine polyphosphates but dramatically blocked the Ca2+ entry induced by ATP and its synthetic analogues. 3 The actions of Ap5A and ATP on the intrasynaptosomal Ca21 increase did not cross-desensitize. 4 Concentration-response studies for diadenosine polyphosphates showed pD2 values of 54.5 ± 4.2 uM and 55.6 ± 3.8 yM for Ap4A and Ap5A, respectively. 5 The entry of calcium induced by diadenosine polyphosphates could be separated into two components. The first represented a selective voltage-independent Ca2+ entry; the second, a sustained phase which was voltage-dependent. 6 Studies on the voltage-dependent Ca2 +-channels involved in the effects of the diadenosine polyphosphates, demonstrated that o-conotoxin G-VI-A inhibited the sustained Ca2+-entry, suggesting the participation of an N-type Ca2+-channel. This toxin was unable to abolish the initial cation entry induced by Ap4A or ApNA. co-Agatoxin IV-A, tetrodotoxin, or nifedipine did not inhibit the effects of the diadenosine polyphosphates. 7 The effect of ATP on Ca2+-entry was abolished by nifedipine and co-conotoxin G-VI-A, suggesting the participation of L-and N-type Ca2"-channels in the response to ATP.8 These data suggest that Ap4A, Ap5A and ATP activate the same intracellular Ca21 signal through different receptors and different mechanisms. Ap4A and Ap5A induce a more selective Ca2+-entry in a voltage-independent process. This is the first time that a selective action of diadenosine polyphosphate through receptors other than PI and P2-purinoceptors has been described.