The respiration physiology of active and dormant land snails was examined in relation to the reduced rates of metabolism and water loss that occur during dormancy. In active snails, partial pressures of O₂ (PO₂) and of CO₂ (PCO₂) in the lung differed by less than 16 torr from air. Arterial PCO₂ was similar to that in lung gas, but arterial PO₂ was as much as 60 torr lower than that in the lung, indicating strong diffusion limitation of O₂ uptake across the lung epithelium. With increase in temperature, PCO₂ increased and PO₂ decreased in the lung, and hemolymph pH declined. During dormancy, mean PO₂ in the lung was 100 torr lower and mean PCO₂ was 50 torr higher than during activity. PO₂ dropped as low as 2.6 torr and PCO₂ rose as high as 101.0 torr. Low PO₂ and high PCO₂ in the lung and hemolymph were associated with low O₂ consumption and also with low values of the respiratory exchange ratio ($R_{E}$), indicating that CO₂ accumulates when O₂ consumption is reduced. Most respiratory CO₂ was released during relatively infrequent periods of high $R_{E}$ and rapid gas exchange. Reduced metabolic rate, high PCO₂, and low PO₂ in the lung are shown to be related to the low rate of evaporative water loss characteristic of dormancy.