The steady-state responses to hypoxia and hypercapnia were studied in freely diving Amazonian manatees (Trichechus inunguis). Breathing room air, the manatees had an end-dive alveolar O₂ Of 60 mm Hg and an alveolar CO₂ of 71 mm Hg. Minute ventilation increased linearly with inspired CO₂ during the hypercapnic breathing and the animals maintained a constant alveolar CO₂. No response to the inhalation of hypoxic mixtures was observed until the alveolar O₂ was less than 30 mm Hg, following which ventilation increased markedly. The increase in minute ventilation during both the hypoxic and hypercapnic testing was primarily due to an increase in breathing frequency which corresponded to a decrease in dive time. It is concluded that CO₂, not O₂, is the important factor in the control of ventilation and diving in the manatee.