The changes in the volume of the cuffs of 200 air-inflated tracheal tubes were studied on exposure to 70% nitrous oxide and 30% oxygen. The length of the cuff, its diameter, wall thickness and residual volume were measured. Increases in the volume of gas occurred in all tubes in a time-dependent manner. The tracheal tubes with low-pressure cuffs were more permeable to nitrous oxide and oxygen than those with high-pressure cuffs. Implications of these findings and the factors which govern the transmission rates of gases through cuff material are discussed.