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1. Factors influencing the glutamate sensitivity of extrajunctional regions of innervated and denervated locust muscle fibres have been investigated. Properties of the two types of extrajunctional glutamate receptors, D- and H-receptors, have been studied in regions of high and low sensitivity. 2. The low level of extrajunctional sensitivity which is normally present in innervated fibres was 20-30 times higher at the muscle-tendon junction than at other sites; increased sensitivity extended about 20-40 micron from the tendon. After denervation or localized damage the entire extrajunctional sensitivity was increased approximately 100 times above control levels. 3. Applying L-glutamate (which activates D- and H-receptors) and DL-ibotenate (which activates H-receptors) from multibarrelled micropipettes showed that increased extrajunctional sensitivity resulted from an increase in D-receptors while H-receptors were apparently unchanged. 4. Coulomb dose vs. response relationships for the action of glutamate on D-receptors were similar when obtained at the muscle-tendon junction and nerve-muscle junction of innervated fibres or at extrajunctional regions in denervated fibres. 5. Time course of onset and percentage desensitization of D- and H-receptors in innervated fibres were similar. The percentage desensitization of D-receptors in extrajunctional regions of high sensitivity was greatly reduced. 6. It is suggested that D- and H-receptors are independent and that the trigger for increased receptor sensitivity acts specifically on D-receptors. In all respects so far studied, the D-receptors resemble extrajunctional ACh-receptors in vertebrate muscle.