LTP is a form of activity-dependent synaptic plasticity that has been investigated mainly in the hippocampus. It is considered likely that similar mechanisms may also account for aspects of naturally occurring plasticity in the neocortex. Consequently, an increasing number of studies have been devoted to the investigation of neocortical LTP. Recent results suggest that at least two forms of LTP coexist in layer III of the neocortex. One depends on NMDA-receptor activation and resembles the LTP observed in hippocampal field CA1. A second form is independent of NMDA receptors and requires activation of voltage-sensitive Ca2+ channels.