1. The effects of Na, Ca and membrane potential on desensitization of postjunctional glutamate receptors on locust muscle were investigated. 2. The kinetics of desensitization were measured ionophoretically. 3. Replacement of Na by equimolar concentrations of the permeant cations Li, NH4 and guanidine and the impermeant cation choline accelerated desensitization onset, increased the steady-state leve of desensitization and reduced the rate of recovery from desensitization. 4. Desensitization onset rates and steady-state levels of desensitization were not significantly altered either by changing the extracellular Ca concentration or by changing the membrane potential of voltage clamped muscle fibres.