On rat cortical slices, cyclothiazide, 1-100 muM, (ED50 = 7.1 +/- 1.1 muM) enhanced the depolarizing action of alpha-amino-3-hydroxY-5-methylisoxazole-4-propionate (AMPA) but not that of N-methyl-D-aspartate (NMDA). Cyclothiazide 10 muM also reversed the action of a 2,3-benzodiazepine, GYKI 53655, which is a non-competitive AMPA receptor antagonist, but not that of the quinoxalinedione, NBQX, which is a competitive AMPA receptor antagonist.