An iontophoretic study of respiratory-related neurons (RN) was conducted in the medullary ventral respiratory area of chronically implanted, undrugged cats during states of sleep and wakefulness. Most RN recorded were unaffected by sleep-wake states but a few RN decreased their firing rate during sleep (sleep sensitive cells). The excitability of RN was assessed in the different states by local application of L-glutamate. Glutamate iontophoresis revealed the presence of 5 cells which were silent during sleep and completely or mostly silent during undisturbed wakefulness but always discharged with a respiratory-modulated pattern of the expiratory type in response to glutamate application. Arousing stimuli induced spontaneous firing of these cells and REM sleep reduced glutamate effectiveness. It was concluded that silent RN and RN which become inactive during sleep permanently receive subthreshold respiratory-modulated inputs which are amplified or depressed by state-dependent tonic inputs.