AbstractAn inhalation exposure of male rats to 300 p.p.m. of a commercial turpentine 6 hrs daily 5 days a week for 8 weeks enhanced the activities of drug biotransformation enzymes of liver microsomes considerably. The activities of NADPH cytochromecreductase and 7‐ethoxycoumarin deethylase, and microsomal content of cytochrome P‐450 were increased 35–60 % during the first weeks of the experiment, but had a tendency to return towards the control values later on. A similar enhancement of activities was also found in liver microsomal epoxide hydratase and UDPglucuronosyltransferase, but these enzyme activities tended to adapt less during the experiment. The turpentine treatment increased the affinity of liver microsomal cytochrome P‐450 to α‐pinene (the main component of the turpentine). The present data suggests that exposure to turpentine is able to modify considerably the biotransformatio