Pentane measurement in exhaled gases was proposed as a safe method to evaluate the importance of lipoperoxidation in vivo, in man and in animals. However we have observed that pentane, which arises from lipoperoxide decomposition, is significantly consumed by cytochrome P-450-rich liver microsomes. This pentane consumption is completely inhibited after heating the microsomes at 100 degrees C, and is considerably reduced by metyrapone, a cytochrome P-450 inhibitor. Hepatic metabolism of pentane, particularly after cytochrome P-450 induction, constitutes a risk of error, when pentane exhalation is taken as an index of lipoperoxidation in vivo.