N-methyl-d-aspartate (NMDA) receptors play crucial roles in several forms of long-term changes in the efficacy of glutamatergic synaptic transmission. The suggestion that the NR2A subunit of the NMDA receptor may be selectively involved in the induction of long-term potentiation (LTP) in the hippocampus and cortex has been challenged. However, the contribution of NR2B in the induction of LTP is not always clearly established. The present study investigates the role of NR2A and NR2B in the induction of LTP in the nucleus accumbens (NAc), a brain region that expresses high levels of NR2B and an NMDA-dependent form of LTP. We recorded extracellular field excitatory postsynaptic potentials/population spikes in slices of mouse NAc. High-frequency stimulation of glutamatergic fibers consistently induced LTP of the field excitatory postsynaptic potential/population spike in the NAc. LTP was abolished in the presence of selective antagonists of either NR2B [R-(R*,S*)-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenyl-methyl)-1-piperidine propanol and Ifenprodil] or NR2A ([(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid) subunits. Recordings performed in a low concentration of Mg(2+) ions in the perfusion solution did not reveal a selective involvement of a particular NMDA receptor subunit because either NR2A or NR2B antagonists were able to block LTP. LTP was also abolished in the presence of a low concentration of the non-subunit-selective NMDA receptor antagonist dl-2-amino-5-phosphonopentanoic acid in normal Mg(2+) and low Mg(2+) in the perfusion solution. These results show that the degree of NMDA receptor activation, and not their subunit composition, determines whether LTP is induced in the NAc.