1 Responses to kainate (KA), willardiine and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were recorded from rat brain mRNA-injected Xenopus laevis oocytes by use of a two-electrode voltage clamp. 2 Thiocyanate (SCN-; 50 microM-4 mM) ions reversibly and selectively inhibited the membrane current responses to AMPA in a non-competitive manner without affecting KA or willardiine-induced responses. 3 The inhibition of AMPA-induced responses by SCN- was dependent on the SCN- concentration with an estimated IC50 of 1 mM. The antagonism was not dependent on the AMPA concentration. 4 The response to a high concentration of AMPA (100-200 microM) exhibited a peak inward current which declined to a steady-state. SCN- inhibited the steady-state current more than the peak response. The inhibition was unaffected by prior incubation with concanavalin-A (Con-A; 10 microM). 5 Responses to KA were antagonized by AMPA in a competitive manner, suggesting that both agonists may activate a common receptor-channel complex. This interaction between two non-NMDA agonists was not affected by the SCN(-)-induced inhibition of the AMPA response. 6 AMPA-induced responses recorded from large cultured cerebellar neurones by whole-cell recording were also inhibited by SCN- in a non-competitive manner. The AMPA-induced peak current was less affected than the steady-state response. 7 We conclude that SCN- can inhibit the response to AMPA in expressed non-NMDA receptors in Xenopus oocytes and also in native receptors in cultured cerebellar neurones. One possible mechanism of action for SCN- inhibition of responses to AMPA may involve a Con-A-insensitive, non-NMDA receptor-mediated desensitization.