Microinjections of kainic acid into the midbrain reticular core were performed in chronically implanted, unanesthetized cats. The immediate effects of kainate excitation were studied during the first 8 h, in animals without any behavioral or EEG signs of epilepsy. Animals displayed pupillary dilatation, piloerection, accelerated respiration, a frozen attitude with a complete lack of facial expression, and no or only very slight orienting reactions. The most structural syndrome was a hallucinatory-type behavior that began in the first hour following the injection. Animals moved forward in a crouched position as if stalking a prey, vocalizing and opening their mouth in an attacking attitude, or moved back as if defending themselves against as imaginary menacer, seeming virtually terrified. The EEG desynchronization began 20-30 sec after the onset of injection and lasted for 12-14 h without any trace of alpha rhythm, spindles or slow waves. Control injections of buffer solution into the midbrain core and kainic acid in other cerebral structures were followed neither by the hallucinatory defense-attack syndrome, nor by comparably long-lasting EEG desynchronization. The hallucinatory-type behavior elicited during the waking state in the present experiments is compared to the oneiric behavior described by Jouvet and Delorme  during paradoxical sleep in animals with suppression of muscular atonia, and possible common mechanisms are discussed.