Inhibition of desmoneme and stenotele nematocyst discharge occurs when Hydra attenuata are fed to repletion. Inhibition can be induced by the application of prey homogenates in the external medium. The onset of inhibition is relatively rapid (<30 s) while the release from inhibition is much slower (>20 min). Inhibition is concentration-depedent. Gel chromatography separation of homogenate shows that the inhibitory substance(s) have a molecular weight greater than 5000. These substances cause the strongest stenotele inhibition and are least effective in activating the feeding reflex (mouth opening and tentacle concerts) which is caused by smaller molecular weight substances. The receptor sites for the inhibitory substance(s) are located on the external surface of the hydra tentacle. Accumulation of prey substances may be the mechanism by which stenotele discharge is inhibited when hydra are fed to repletion.