1. 1. Responses evoked by single or rhythmic clicks at the level of cortical and subcortical, specific and nonspecific structures, have been recorded in experiments performed on encéphale isolé cats (curarized and uncurarized animals). Fast repetitive stimulation of the reticular formation (FR) and mid-line thalamic nuclei exerts characteristics effects, the sense of these influences depending on the click-frequency. 2. 2. By simultaneously recording at several points in the auditory cortex, it has been shown that the diminution of acoustic response to a single mono- or binaural click, during electrocortical arousal induced by FR stimulation, is not a global event, common to all ectosylvian areas. The most striking suppression of acoustic potential occurs at the level of the antero-superior region of area I (EAS). On the other hand, a marked facilitation could be often revealed on the response to a single click in the dorsal region of area II (adjacent to area I), simultaneously with the suppression of the potentials evoked by the same stimulus in other areas of the auditory cortex. Even when the surface-positive potential decreases in area II, the negative one is much increased during FR stimulation. The possibility is suggested that a double action, in opposite senses, is exerted by the reticular system on depolarizing processes at the level of basal and apical dendrites. This phenomenon (simultaneous suppression and facilitation of potentials recorded in various points of the same sensory area) is described under the title commutation of exitability representing a temporary transfer of excitability (under reticular stimulation) from one cortical point to another neighbouring one. This might be interpreted in the sense of positive and negative induction taking place at the cortical level during conditioned processes. 3. 3. Study of FR effects on the capacity of cortical auditory areas to respond to paired clicks at 50 msec intervals, demonstrated a marked facilitatory influence of reticular arousal, manifested by a decrease in the refractory phase of acoustic responses. 4. 4. The polymorphous effect of reticular stimulation on cortical potentials evoked by low-frequency rhythmic clicks is accompanied (in encéphale isolé curarized cat) by obvious signs of facilitation of acoustic responses recorded from the medial geniculate body, reticular thalamic nucleus and n. lateralis posterior. 5. 5. From rates of 5/sec upwards, reticular facilitation of potentials evoked by rhythmic clicks is extended to all recorded areas from the auditory cortex. In some experiments, the rate at which responses to clicks suffer a global facilitation is much higher (up to 30–35/sec); such cases were generally found in alert animals, exhibiting protracted arousal (induced or spontaneous) reactions. Facilitation mainly affects the negative component of fast rhythmic cortical responses which become appreciably ampler and steeper; enhancement of the surface-negative potentials is a certain sign of direct intracortical facilitation elicited by the reticular ascending system. Facilitation of responses in deep (specific and unspecific) structures, already present for low rate stimulation, persists up to the highest rates used. 6. 6. Habituation of acoustic potentials generally takes place, at first, in the cortical auditory areas and later in the medial geniculate body. Reticular stimulation causes responses to recover their initial amplitude, inducing an increase of the negative component, sometimes becoming even greater than prior to habituation. 7. 7. The processes related in the present paper generally confirm our previous data concerning responses to flickering light. The greater complexity of reticular facilitation phenomena in the auditory system is to be found in the morpho-functional peculiarities of acoustic structures.