The present study describes a modification of the fast drug-application technique (ticker) which combines two fast-application systems, ‘the double-ticker’. With the double-ticker, drugs can be applied to excised patches from either one of the tickers permitting switching among three different solutions in the sub-millisecond range. We made use of this advantageous feature of the double-ticker to study two aspects of the glutamate receptor channel in crayfish muscle. The first concerns revealing the number of glutamate binding sites from measurements of a dose-response relation (2–3 sites). The other relates to the state from which the receptor undergoes desensitization. For the quisqualate-sensitive glutamate receptor desensitization occurs from a closed state. This is in addition to desensitization from an open state.