Mammalian dendrites are active structures capable of regenerative electrical activity. Using fast confocal imaging in combination with low affinity calcium indicator dyes we have investigated the characteristics and function of local dendritic spikes in layer 5 pyramidal neurons of mouse visual cortex. We found that these dendritic spikes initiate a fast calcium transient in a small spine-dendritic compartment. Furthermore, a single of these dendritic spikes induced long-term synaptic depression. This form of activity-dependent synaptic plasticity did not require somatic spiking. However, the co-incident activation of synaptically-induced local dendritic spikes and back-propagating action potentials produced long-term potentiation (LTP). Even a single pairing event was sufficient for LTP induction, provided that the interval the somatic action potential occurred during a brief time window (of about 50 ms) after the induction of the local dendritic spike. This critical time window was characterized by a superlinear increase in amplitude of the intradendritic calcium transient. A low amplitude calcium transient induced LTD, whereas a high amplitude calcium transient induced LTP. Remarkably, these local dendritic calcium transients had a similar time-course. In conclusion, we identified a new form of rapidly-induced bidirectional synaptic plasticity. We propose that single-shock LTP and LTD underlie the rapid acquisition of information in cortical circuits.