A distinctive characteristic of the AMPA subset of glutamate receptor channels is their remarkably rapid desensitization. A family of compounds, the benzothiadiazides, is described here that potently inhibit rapid glutamate receptor desensitization. The structure- activity relationships of these compounds are examined and the actions of cyclothiazide (CYZ), the most potent of these compounds, are described in detail. At the macroscopic level CYZ reduced rapid desensitization, enhancing the steady-state and peak current produced by 1 mM quisqualate with EC50 values of 14 and 12 microM, respectively, and shifted the quisqualate peak current concentration-response relation to the left. The slight outward rectification of the steady- state quisqualate current-voltage relationship was reduced by CYZ. At the microscopic level CYZ caused glutamate to induce long bursts of channel openings, and greatly increased the number of repeated openings. At 10 microM CYZ did not have measurable effects on the fast component of deactivation nor did it have statistically significant effects on the distribution of the faster components of glutamate- induced burst duration. In contrast, 10 microM CYZ increased the amplitude and significantly prolonged the duration of the spontaneous miniature EPSCs. The identification and characterization of this new family of gating modifiers may further facilitate the investigation into the mechanisms underlying rapid glutamate receptor desensitization and the physiological roles that it may serve.