The cytochrome P-450-dependent metabolism of n-pentane was studied by head-space gas chromatographic analysis of incubations with liver microsomes and reconstituted systems from rat and rabbit liver. The alkane was metabolized in liver microsomes from imidazole-treated rabbits at an apparent Vmax of 3 nmol mg-1 min-1 and a Km of 35 microM and in liver microsomes from acetone-treated rats at an apparent Vmax of 10 nmol mg-1 min-1 and a Km of 9 microM. The rate of microsomal n-pentane metabolism was enhanced fivefold by acetone treatment of rats, compared to the rate observed in liver microsomes from control rats. The reaction was inhibited in microsomes by compounds that are known to interact with the acetone and ethanol-inducible form of liver microsomal cytochrome P-450, like acetone, imidazole, ethanol and benzene. Effective inhibition was also accomplished when microsomal incubations were performed in the presence of IgG against this form of cytochrome P-450 from either rat or rabbit liver. In reconstituted membrane vesicles containing NADPH--cytochrome P-450 reductase, ethanol-inducible P-450 from rat liver (P-450j) was a fivefold more efficient catalyst of pentane metabolism than was the corresponding P-450 form from rabbit liver (P-450 LMeb). The rabbit enzyme metabolized the hydrocarbon with an apparent Vmax of 4 nmol nmol-1 min-1 and a Km of 8 microM, By contrast, phenobarbital-inducible P-450 LM2 or 3-methylcholanthrene-inducible P-450 LM4 from rabbit liver were quite ineffective catalysts of n-pentane metabolism. It is concluded that n-pentane constitutes a good substrate for cytochrome P-450 and, in particular, for the ethanol-inducible form of this hemoprotein. It is suggested that n-pentane is an important substrate for P-450 under in vivo conditions and that pentane measurements in expired air as an indicator of lipid peroxidation must be interpreted with caution.