High affinity extrasynaptic NMDA and GABA_A receptors both can detect ambient concentrations of endogenous agonists. GABA_A receptor mediated tonic current is present at physiological conditions while tonic current mediated by NMDA receptors could be obtained only when receptors are relived from voltage-dependent channel block by Mg²⁺. Using two-photon calcium imaging in slices we demonstrated that extrasynaptic NMDA receptors in dendrites of hippocampal pyramidal neurons are pre-bound to ambient glutamate and mediate a proportion of calcium transients during backpropagating action potentials. This happens because of transient relief of Mg²⁺ block at the time of action potential. Local glutamate uncaging paired with backpropagating action potential further increased NMDA receptor dependent part of calcium transient. Such 'extrasynaptic coincidence detection' could be important for synchronisation of simultaneously firing neurons. In other words glutamate spillover from neighbouring synapses or glutamate release by astrocytes can be sensed in larger extent by those neurons which fire at the same time.