A simple and accurate quantitative assay of the glutathione-activated feeding reflex in Hydra is described. The results show: (a) There are a limited number of receptor-effector systems, probably localized in the area immediately around the mouth and on the tentacles. (b) Concentrations of glutathione greater than 5 x 10-6 M activate all these systems; 10-5 M glutathione elicits a half-maximum response. (c) Glutathione must be constantly present at the receptor site in order for a response to occur. (d) The response stops in the presence of excess glutathione because of some change within the Hydra, and not as a result of any alteration to the glutathione. The present state of knowledge concerning the mechanism by which glutathione combines with and activates the glutathione receptor to elicit the feeding reflex is summarized.

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