The control by precurrent sensory stimulation of the side to which Automeris first swings its head at the beginning of a rocking response (the bias of the response) was examined. The responses of some moths are always biased to the same side whatever stimuli are presented. The direction of the bias of other moths can be controlled by bilaterally asymmetric stimuli which themselves do not release a response. When rocking is elicited after unequal, sustained illumination of the eyes, the response is biased to the more brightly lit side. The response is also biased to the same side as a unilateral ‘on’ stimulus and less consistently to the opposite side from a unilateral ‘off’ stimulus. The following results imply that the action of light and of ‘on’ stimuli on rocking are excitatory, whereas the action of ‘off’ stimuli is inhibitory: (1) Intense ‘on’ stimuli evoke the rocking response more frequently than intense ‘off’ stimuli; (2) the number of oscillations in a response (rocking score) is greater if moths are kept under bright illumination than if they are kept under dimmer illumination; and (3) an ‘off’ stimulus delivered during the course of a response will cut it short. The opposite actions of ‘on’ and ‘off’ stimuli can also be seen in the bias of the response after flashes of different lengths. After a long flash (500 msec) the response is biased to the same side as the flash, but after a short flash (5 msec) the response is more commonly biased to the opposite side from the flash. The flash length at which the direction of the bias switches is related to the excitability of the system emdiating rocking. Thus the responses of moths with high rocking scores are biased ipsilaterally by flashes which impose a contralateral bias on the response of moths with lower rocking scores. Stimulation with identical, overlapping flashes to both eyes showed that an ‘on’ stimulus to one eye inhibits the effects of a slightly later ‘on’ stimulus to the other eye. Proprioceptive stimuli also control the direction of the bias. The after-effects of visual and proprioceptive stimuli probably decay at different rates and may be stored simultaneously. The results are discussed with reference to the electrical activity of visually sensitive units recorded in the medial protocerebrum of saturniid and sphingid moths.