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The responses of crayfish muscle fibres to bath application or long ionophoresis of L-glutamate were studied in normal and low Ca2+ solutions. The smaller responses recorded in low Ca2+ solutions have characteristics suggesting a faster desensitization. Desensitization and recovery have complex kinetics. Desensitization is faster and recovery slower when external Ca2+ concentration is reduced. Both components of the recovery phase, which can be fitted by the sum of two exponentials, are affected by the external Ca2+ concentration. Recovery can be accelerated by external Ca2+ ionophoresis onto desensitized glutamate receptors. Responses to brief glutamate pulses of low intensity are not affected by Ca2+ reduction. For higher intensities, signs of desensitization are detectable early in the rising phase of the response. Concanavalin A (Con A) blocks both desensitization and Ca2+ dependence with similar time courses. Whether or not the preparation has been treated with Con A, the slowly rising responses recorded in isotonic Ca2+ do not show signs of desensitization. Con A causes a partial blockade of the glutamate response. The Ca2+ dependence of the glutamate response can be explained by the Ca2+ dependence of the desensitization process, the cation acting at ectocellular sites of the muscle membrane.