To evaluate the effect of straight-chain alkanes on normal detoxication reactions, we studied the in vitro effect of the homologous series n-hexane through n-dodecane on two cytochrome P-450 (EC 1.14.14.1) enzyme activities. Benzo[a]pyrene hydroxylase (BaPOHase) and 7-ethoxycoumarin deethylase activities were measured in liver and lung microsomes of control and beta-naphthoflavone-treated rats. In the presence of 2 mM n-hexane through n-dodecane, liver BaPOHase activity decreased from 67% of control with n-dodecane to 21% of control with octane. Lung benzo[a]pyrene hydroxylase was insensitive to all tested alkanes at 2 mM. In the presence of 2 mM alkanes, liver 7-ethoxycoumarin deethylase activity decreased from 73% of control with n-octane to 28% with n-octane. Lung 7-ethoxycoumarin deethylase was also sensitive to the alkane series. In the presence of 2 mM alkane the greatest effect was obtained with n-octane and represented a 56% loss in activity. Alkane concentration-dependence measurements showed 0.02-0.20 mM as the sensitive region of the curve for n-octane with maximal loss of activity achieved at 0.20 mM. Liver ethoxycoumarin deethylase activity from beta-naphthoflavone-treated rats was less sensitive towards the reactive alkane, n-octane, than the activity from control rats. Double-reciprocal-plot analysis revealed the maximal velocity (Vmax) was decreased in the presence of 0.2 mM n-octane. Hence this hydrocarbon did not exert its effect solely as an alternate substrate. The data show the n-alkanes, n-hexane through n-dodecane, interfered with a normal detoxication pathway in a manner that was chainlength-dependent, tissue-specific, and dependent on the preexposure history of the animal.