The plasticity of synapses within neural circuits is regulated by activity, but the underlying mechanisms remain elusive. Using the dye FM1-43 to directly image presynaptic function, we found that large numbers of presynaptic terminals in hippocampal cultures have a low release probability. While these terminals were not readily modifiable, a transient but not permanent long-term reduction of network activity or Ca2+ influx could increase their modifiability. This modulation of plasticity was mediated by Ca2+ flux through NMDA and voltage-gated calcium channels and was lost within 48 hr. A more permanent enhancement of synaptic plasticity was achieved by selectively reducing the Ca2+ flux associated with uncorrelated activity via adjustment of the voltage-dependent Mg2+ block of the NMDAR. Upregulation of NR2B-containing NMDARs induced by this treatment is an important but not sole contributor to the enhancement of plasticity. Thus, quantity and quality of activity have differential effects on the intrinsic plasticity of neurons.