Abstract: Disclosed is an isocapnic respiratory stimulator for providing a mixture of fresh air and CO2 enriched exhaled air. The respirator comprises a mixing chamber with a breathing port and at least one vent port. Fresh air is mixed thoroughly with exhaled air in the mixing chamber. Upon sustained breathing from the chamber, the minute ventilation of the insure is increased with experiencing any substantial change in arterial blood CO2 level. This provides methods to avoid or treat hypoxia experienced by an individual at altitudes above 5000 feet, to treat carbon monoxide poisoning, to induce losing weight, to assist in smoking cessation, to condition an athlete to minimize dyspnea, and to condition respiratory muscles.