We have investigated the motor and ponto-geniculo-occipital (PGO) wave response to startle eliciting stimuli in the unanesthetized cat. We found that the amplitude of the PGO spike recorded in the lateral geniculate nucleus (LGN) increases monotonically with increasing intensities of auditory stimuli. In contrast, the motor response to low intensity (< 75 dB) stimuli is characterized by electromyographic (EMG) suppression, while at higher intensities and EMG excitation is superimposed on this suppresion. Thus PGO elicitation is accompanied by EMG suppression at low intensities and by a net EMG excitation at high intensities. While the amplitude of the auditory elicited PGO response is a graded function of stimulus intensity, somatic stimuli tend to elicit the PGO response in an all-or-none fashion. Both the motor and PGO responses to sensory stimulation change with behavioral state. The EMG suppression by auditory stimulation increases in duration during the transition to rapid eye movement (REM) sleep. Elicited PGO amplitude is highest in transitional sleep, lower in quiet waking and REM sleep and lowest in active waking. Prepulse inhibition of PGO spikes is greatly attenuated during transitional and REM sleep. We hypothesize the existence of 3 phasic response systems, a motor suppression system, a motor excitation (startle) system and a PGO elicitation system. While these systems are triggered concurrently by intense phasic stimuli in waking, they are modulated independently by stimulus intensity and behavioral state, and have different rates of habituation. These systems act in concert to produce behavioral responses to suddent onset stimuli.