alpha-Amino-3-hydroxy-5- methylisoxazole-4-propionate (AMPA) is an excitatory amino acid which on the basis of electrophysiological and binding studies appears to act as a quisqualic acid receptor agonist. AMPA and other excitatory amino acids, such as quisqualic acid, kainic acid, and N-methyl-D-aspartic acid, as well as picrotoxin, an inhibitor of endogenous GABA, produce a marked stimulation of locomotor activity after bilateral injection into the nucleus accumbens. The intraacumbens injection of gamma-D-glutamylaminomethylsulphonate (GAMS) was found to inhibit the hypermotility responses produced by AMPA and quisqualic acid at doses that were unable to inhibit the hypermotility responses produced by kainic acid, N-methyl-D-aspartic acid, and picrotoxin. These results suggest that GAMS is able to selectively inhibit quisqualic acid receptors in the nucleus accumbens. The intraacumbens injection of D-alpha-aminoadipic acid at a dose that significantly inhibited N-methyl-D-aspartic acid-stimulated locomotor activity did not produce a significant inhibition of AMPA-stimulated locomotor activity, suggesting that AMPA is not acting at N-methyl-D-aspartic acid receptors. Thus, these results suggest that the activation of quisqualic acid receptors in the nucleus accumbens produces a hypermotility response.