The influence of low-frequency rhythmic stimulation of the midbrain reticular formation (RF) on neuronal activity in the visual cortex was studied in waking, non-immobilized rabbits during elaboration of defensive conditioned reflex to light flashes. In most neurones, the RF stimulation acted in the same direction as the painful reinforcement (electrocutaneous stimulation of the limb), potentiating the effect of the latter and contributing to changes of reactions to light flashes which became a signal of defensive response, with an increase of their similarity to reactions to paired stimuli. In some of the neurones RF stimulation produced changes in structure of phase reactions to light flashes, i.e. intensified the first phase of activation, reduced initial inhibitory pause, and augmented the number of periods of activation--inhibition alternation.