The N-methyl-D-aspartate receptors (NMDARs) play a key role in synaptic plasticity, but it remains unclear whether the intrinsic-firing properties, another major determinant of the functional output of neurons, are regulated by activation of NMDARs. Here, we examine the effects of NMDAR activation on the intrinsic-firing properties of medium spiny neurons in nucleus accumbens in vitro. NMDAR activation by bath application of NMDA increased both the intrinsic excitability and the spike adaptation of these neurons. Furthermore, selective activation of NR2A-containing NMDARs mediated the enhancement of spike adaptation, whereas selective activation of NR2B-containing NMDARs increased the intrinsic excitability, suggesting that NR2A-containing and NR2B-containing NMDARs play different roles in mediating the intrinsic-firing properties of neurons.