Repeated tactile stimulation of the siphon in Aphysia normally results in habituation of the gill withdrawal reflex and a concomitant decrease in the amplitude of the excitatory synaptic input ot gill motor neurons in the abdominal ganglion. It was found, however, that induced low-level tonic activity in motor neuron L9, which does not itself elicit a gill withdrawal movement, prevented habituation of the reflex from occurring. Further, in preparations already habituated, this tonic low-level activity brought about a reversal of habituation. Although tonic L9 activity prevented the occurrence of habituation or brought about its reversal, it did not interfere with the synaptic decremental process which normally accompanies gill reflex habituation. Motor neurons L7 and LDG1 were found not to possess this ability of L9 to modulate gill reflex habituation. Evidence suggests that L9's modulatory effect is mediated in the periphery, in the gill and not centrally in the abdominal ganglion.