The modulation of α-amino-3-hydroxy-5-methyl-4-isoxazol-propionate (AMPA) receptor-mediated currents by cyclothiazide was investigated in acutely isolated cells from rat striatum with whole-cell patch-clamp recording. Single-cell reverse transcriptase-polymerase chain reaction (RT-PCR) was used to identify medium spiny and giant aspiny neurons and to determine their AMPA receptor subunit composition mostly in separate experiments. After pretreatment with cyclothiazide, kainate-induced AMPA responses were more strongly potentiated in medium spiny than in giant aspiny neurons; cyclothiazide induced a ninefold leftward shift in the kainate concentration–response curve for medium spiny neurons (not giant aspiny neurons). The EC50s for the cyclothiazide potentiation did not differ substantially between medium spiny neurons and giant aspiny neurons. The recovery of kainate-activated currents from modulation by cyclothiazide was slower for medium spiny neurons than for giant aspiny neurons. Medium spiny neurons expressed GluR-A, GluR-B and GluR-C, but not GluR-D subunits in both flip and flop splice variants. All giant aspiny neurons expressed GluR-A and GluR-D, exclusively in the flop form, half of them also expressed GluR-B and GluR-C. This is in keeping with slow and fast desensitization kinetics in medium spiny neurons and giant aspiny neurons, respectively, and differences in cyclothiazide modulation. The rate of cyclothiazide dissociation from the AMPA receptor, activated by glutamate, was ∼ 90 times slower in medium spiny neurons than in giant aspiny neurons. In giant aspiny neurons (not medium spiny neurons) this rate was strongly dependent on the presence of an agonist; 1 mm glutamate increased it 30-fold. Thus, two major cell groups in the striatum display distinct AMPA receptor compositions carrying specific properties of glutamate responses. Excitatory transmission will thus be differentially affected by cyclothiazide-type compounds.