Diadenosine polyphosphates are a family of dinucleotides formed by two adenosines joined by a variable number of phosphates. Diadenosine tetraphosphate, Ap4A, diadenosine pentaphosphate Ap5A, and diadenosine hexaphosphate, Ap6A, are stored in synaptic vesicles and are released upon nerve terminal depolarization. At the extracellular level, diadenosine polyphosphates can stimulate presynaptic dinucleotide receptors. Responses to diadenosine polyphosphates have been described in isolated synaptic terminals (synaptosomes) from several brain areas in different animal species, including man. Dinucleotide receptors are ligand-operated ion channels that allow the influx of cations into the terminals. These cations reach a threshold for N- and P/Q-type voltage-dependent calcium channels, which become activated. The activation of the dinucleotide receptor together with the activation of these calcium channels triggers the release of neurotransmitters. The ability of Ap5A to promote glutamate, GABA or acetylcholine release has been recently described by the present authors in rat midbrain synaptosomes.