Lipid peroxidation by free radicals has been suggested as a mechanism of a lung injury caused by breathing higher than normal concentrations of oxygen. The appearance of hydrocarbons such as n-pentane in the expired gas of mammals has been proposed asin vivoevidence of lipid peroxidation. The excretion of n-pentane was studied in 15 healthy volunteers in whom excretion of exogenous n-pentane was determined over a 60- to 90-min period while breathing hydrocarbon-free gases. N-pentane elimination rates (mean ± SEM) in the expired gas at 0, 30, 60, 90, and 120 min were 10.2 ± 1.5, 1.6 ± 0.2, 1.2 ± 0.9, 1.3 ± 0.4, and 1.3 ± 0.3 (pmol · kg−1· min−1), respectively. Using a specially assembled circuit, a 2-h oxygen exposure study was performed on six healthy volunteers, in whom basal n-pentane excretion varied ten-fold among individuals, from 0.25 to 2.25 pmol · kg−1· min−1. After breathing 100% oxygen, npentane excretion was augmented 62–420% within 30 to 120 min. The authors conclude that lipid peroxidation may occur in humans within 30 min of breathing 100% oxygen.