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1. The reflex effects on the pattern of breathing and total lung resistance of introducing 30, 10 and 5% CO₂ in air into the larynx have been studied in anaesthetized and decerebrate cats breathing through a tracheostomy tube.

2. Flowing 30% CO₂ into the larynx caused a two-phased response. First, respiratory frequency and tidal volume decreased, with a consequent fall in minute ventilation. After two to ten breaths, frequency remained slow, but tidal volume increased beyond the control level, so that minute ventilation was restored to control levels.

3. Flowing 5 or 10% CO₂ into the larynx caused slowing of breathing with small and inconsistent changes in tidal volume. Minute ventilation was significantly diminished.

4. Off effects, on re-introducing air into the larynx, after 2 and 10 min of CO₂ exposure, suggested that the reflex response diminishes with increased duration of exposure to CO₂.

5. None of the concentrations of intralaryngeal CO₂ changed total lung resistance or compliance.

6. CO₂ mixtures in the larynx generally caused no change in blood pressure or pulse rate of the cats.

7. The reflex effects of intralaryngeal CO₂ were abolished by denervating the larynx.

8. Hypoxic mixtures introduced into the larynx did not change breathing.