The snorkel, which allows swimmers to keep their face down in the water while breathing, is widely used by divers, spear Fishermen and moncifin swimmers. A snorkel adds an additional dead space of 160 - 170 ml and causes an increase in the concentration of CO, in the inspired gas due to expired air trapped in the snorkel which is then re-inspired. In this study the metabolic and the ventilatory response to rebreathing the expired air in the snorkel were investigated in twelve human subjects. A 2900 C Sensor Medics gas analyzer was used in breath-by-breath mode for the measurements. Ventilation (V-E), respiratory rate (RR), tidal volume (TV), oxygen consumption (VO2) and carbon dioxide production (VCO2) were measured at rest and during light exercise both with and without the snorkel dead space. We observed a significant increase in all variables except RR, when subjects rebreathed the gas in the snorkel. The increase in ventilation resulted from an increase in tidal volume rather than increasing respiratory rate. We conclude that the work of breathing is increased when CO2 concentration is high in inspired gas and rebreathing while snorkelling can be prevented by a new snorkel design with a low-resistance two-way non-rebreathing valve, which will allow the expired air flow into the water.