The contraction reponse ot hydra to intermittent light stimulation may be inhibited by exposing the animal to reduced glutathione (GSH). Such inhibitory acthity is dependent on: (1) the concentration of GSH; (2) the pH of the medium; (3) previous exposure to GSH; and (4) the nutritional state of the animal. Hydra adapt to lO−5 M GSH so that after approximately an hour the frequencies of lightinduced contractions are restored to control levels. Such adaptation to GSH is due to changes occurring within the animal rather than to the degradation of the glutathione molecule. S-methyl glutathione blocks contractions in response to light, showing that the sulfhydryl group is not essential for inhibition. Analogs with sterically large groups substituted for the sulfhydryl group, such as oxidized glutathione and S-acetyl glutathione, have no inhibitory activity. These compounds, however, reduce the inhibitory effect of GSH, indicating competition for the GSH receptor. Contractions of hydra in response to intermittent mechanical agitation are also inhibited by GSH. The duration of inhibition is dependent on the GSH concentration. Both oxidized glutathione and S-acetyl glutathione reduce the inhibitory effect of GSH. Hydra adapted to the dark for 24 hours show a marked suppression of contractions in response to mechanical agitation when exposed to light. On exposure to light, such animals elongate to approximately one and a half times their dark adapted length, and are relatively insensitive to agitation. The mechanisms by which such stimuli inhibit the contraction responses of hydra remain to be determined.