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 β‐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 ethoxy‐coumarin deethylase activity from β‐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‐depen‐dent, tissue‐specific, and dependent on the preexposure history of the animal.