The ventilatory response to CO2 was subdivided into that portion due to increasing rib cage expansion, and that due to increased diaphragmatic descent. Five children were studied, awake, and anesthetized with halothane, 0.8–0.9 per cent. During anesthesia there was a 67 ± 8 per cent reduction (mean ± SE) in the slope of the response of overall ventilation to an increase in CO2. This was primarily due to an 89 ± 8 per cent reduction in the recruitment of rib cage ventilation (P < .001). There was no significant change in the slope of the diaphragmatic response (anesthetized value 19 ± 21 per cent less than control), although the response curve was shifted to the right so that a higher CO2 concentration was needed to stimulate a given level of diaphragmatic excursion. Additional measurements of the inspiratory intercostal electromyogram in three adult subjects documented a rapid, profound depression of intercostal activity with halothane anesthesia that was associated with a marked decrease in rib cage ventilation. The authors conclude that a major component of the ventilatory depression associated with halothane anesthesia results from the preferential suppression of intercostal muscle function with relative sparing of diaphragmatic activity.