The effect of sleep state on ventilation and the mechanics of breathing was studied in nine normal adolescents by use of a respiratory inductive plethysmograph and surface electromyogram electrodes. Minute ventilation was state dependent (P less than 0.01), decreasing by a mean of 8% from wakefulness to nonrapid-eye-movement (NREM) sleep and increasing 4% from NREM to rapid-eye-movement (REM) sleep. These changes were caused by changes in respiratory rate. Tidal volume (VT) was not affected by sleep state (P greater than 0.10). The pattern of breathing during wakefulness was similar to that of REM sleep. During NREM sleep intercostal and diaphragmatic muscle activity increased by a mean of 34% and 11%, respectively, as compared with wakefulness, indicating an increase in the respiratory work load. This was accompanied by a substantial increase in rib cage contribution to VT. REM sleep was associated with a marked decrease in intercostal muscle activity (P less than 0.05) and a diminished rib cage contribution; VT was maintained due to a mean increase of 34% in diaphragmatic muscle activity (P less than 0.05).