Intercostal-muscle activity (I-EMG) was studied in sleep and wakefulness in ten adult cats. The animals were implanted with electrodes for recording the electroencephalogram (EEG), electrooculogram (EOG), and pontogeniculooccipital (PGO) waves as well as intercostal- and neck-muscle activity. A small-animal pneumotachograph connected to a tracheal cannula was used to monitor respiration. The data were obtained in 87 recording sessions, each lasting 5-6 h. Total EEG power (fast Fourier analysis) and integrated I-EMG activity were computed over consecutive 5.12-sec periods of wakefulness and nonrapid eye movement (NREM) sleep. Cycle-triggered histograms based on 50 breaths were computed also in wakefulness and NREM sleep. I-EMG was classified as expiratory or inspiratory according to when the activity was maximal during the respiratory cycle. Expiratory I-EMG augmented progressively during NREM sleep with the maximal activity occurring just before rapid eye movement (REM) sleep. The mean correlation between total EEG power and expiratory I-EMG for all expiratory muscles was 0.441 (+/- 0.16 SD; n=24). In contrast to the consistent augmentation of expiratory I-EMG, inspiratory I-EMG changes from wakefulness to deep NREM sleep varied within and between muscle groups: in 21% of the cases, activity decreased from wakefulness to NREM sleep; in 16% of the cases, activity increased; in 56% of the cases, activity did not change; and in 7% of the cases, activity increased in only the expiratory phase. We conclude the consistent augmentation of expiratory intercostal muscle indicates an active, NREM sleep-specific respiratory process that may have important implications for lung mechanics in that state.