Many aspects of dopamine (DA) systems mature during adolescence. In the nucleus accumbens, the modulation of prefrontal cortical synaptic responses by DA becomes refined during adolescence with the recruitment of a gamma-amino butyric acid (GABA) component. As this GABA component is depolarizing, it remains to be determined whether this change affects action potential firing in nucleus accumbens neurons. Here we tested whether a D2 agonist affects AMPA-evoked cell firing in slices containing the nucleus accumbens from juvenile (postnatal day, PD 28–34) and adult (PD > 60) rats. 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) (0.1–0.4 μM) depolarized nucleus accumbens neurons and increased their firing in a dose-dependent manner. The D2 agonist quinpirole (2 μM) had different effects in juvenile vs. adult slices. In the juvenile accumbens, quinpirole enhanced AMPA (0.2 μM) effects on evoked firing in a subset of neurons while it had no effect on the rest. In the adult accumbens, the D2 agonist instead attenuated the effect of AMPA on evoked firing, an interaction that was blocked by the GABA-A antagonist picrotoxin (50 μM). Thus, D2 receptors modulate AMPA responses in the nucleus accumbens differently in juvenile than adult rats, and the adult effect requires local GABA transmission. The incorporation of a GABA component in the modulation of information processing in the nucleus accumbens by DA during adolescence may allow for a better contrast in cortically activated ensembles. Synapse, 2012. © 2011 Wiley Periodicals, Inc.