Adenosine 5′-triphosphate (ATP) stimulates a [Ca 2+ ] i increase via specific ionotropic receptors, termed P2X receptors, in rat midbrain presynaptic terminals. A microfluorimetric technique enabled study of the [Ca 2+ ] i increase in isolated single synaptic terminals, showing that 33.4±2.5% of them responded to ATP. Immunological studies carried out, after functional studies, with specific anti-P2X receptor subunit antibodies showed only positive labelling with anti-P2X 3 antibodies in 23.5±1.7% of the terminals. All positively P2X 3 labelled synaptic terminals responded to ATP. Nevertheless, not all of them responded to α,β-meATP, these representing 6.7±1.5% of the total. In addition, 9.8±2.3% of the terminals responded to ATP but exhibit negative P2X 3 -labelling. These results demonstrate the existence of a heterogeneous population of ionotropic ATP receptors at the presynaptic level.