Ca(2+)-sensitive kinases are thought to play a role in long-term potentiation (LTP). To test the involvement of Ca2+/calmodulin-dependent kinase II (CaM-K II), truncated, constitutively active form of this kinase was directly injected into CA1 hippocampal pyramidal cells. Inclusion of CaM-K II in the recording pipette resulted in a gradual increase in the size of excitatory postsynaptic currents (EPSCs). No change in evoked responses occurred when the pipette contained heat-inactivated kinase. The effects of CaM-K II mimicked several features of LTP in that it caused a decreased incidence of synaptic failures, an increase in the size of spontaneous EPSCs, and an increase in the amplitude of responses to iontophoretically applied alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate. To determine whether the CaM-K II-induced enhancement and LTP share a common mechanism, occlusion experiments were carried out. The enhancing action of CaM-K II was greatly diminished by prior induction of LTP. In addition, following the increase in synaptic strength by CaM-K II, tetanic stimulation failed to evoke LTP. These findings indicate that CaM-K II alone is sufficient to augment synaptic strength and that this enhancement shares the same underlying mechanism as the enhancement observed with LTP.