Larval lampreys (ammocoetes) exhibit a rapid vibration-evoked startle response involving a bilateral activation of musculature along the length of the body. The resulting movement is variable, contingent on the animal's prestimulus posture: lateral curves along the trunk and tail contract more on the inner side of the curve than on the outer side. Thus, the startle response increases preexisting body curvature. Because ammocoetes are burrowing filter feeders, this startle behavior results in rapid withdrawal of the head into the burrow. A vibratory pulse to the otic capsules in a semi-intact preparation evokes simultaneous action potentials in both primary Mauthner neurons. Vibration also excites several Müller cells. Intracellular stimulation of one primary Mauthner axon (eliciting one action potential) produces bilateral trunk electromyographic potentials that are smaller than those evoked by vibration; simultaneous stimulation of both Mauthner axons (one action potential each) reproduces the vibration-evoked electromyographic amplitudes. The Mauthner cell's sensitivity to vestibular input is centrally modulated during changes in behavioral state. Mauthner action potentials are most easily elicited by vibratory or electrical stimulation of vestibular afferents while an intact animal is at rest; the same stimuli become subthreshold for Mauthner activity while the animal is swimming. A similar depression of Mauthner excitability is observed in semi-intact preparations during arousal. 'Arousal' was defined by the occurrence of tonic, descending spinal cord discharge. Mauthner cells are tonically depolarized during arousal and exhibit an increased membrane conductance; excitatory postsynaptic potentials evoked by vibratory or electrical stimulation of vestibular afferents are greatly attenuated. Modulated sensory transmission to the Mauthner cell may help to prevent inappropriate activation of the startle circuit.