Rapid acquisition of information is thought to involve neocortical neurons. The cellular mechanisms of this process are largely unknown. Experiments from our laboratory indicated that local dendritic spikes may boost the induction of long-term depression (LTD and long-term potentiation (LTP) in cortical neurons, and, thus, accelerate the induction process. We found that synaptically-evoked local dendritic spikes initiate a fast calcium transient in a small spino-dendritic compartment. Our results indicated that a single of these dendritic spikes is sufficient to induce LTD (Holthoff et al., J. Physiol., 2004). In contrast to other forms of activity-dependent synaptic plasticity, this LTD did not require somatic spiking. Surprisingly, the co-incident activation of synaptically-induced local dendritic spikes and back-propagating action potentials produced 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 supralinear increase in the amplitude of the intradendritic calcium transient. Taken together, our results reveal a new form of rapidly-induced bidirectional synaptic plasticity. We propose that this instantly-induced LTP and LTD underlie the rapid acquisition of information in cortical circuits.