Many factors influence the metabolism of drugs in man. Besides genetic factors, environmental factors may play a significant role in explaining the variation observed in the rates of drug metabolism between different individuals. Intentional or unintentional exposure to environmental chemicals could enhance or inhibit the activity of hepatic mixed function oxidases that metabolise drugs and other foreign chemicals, as well as endogenous substrates such as steroid hormones. A major source of such exposure may be occupational. Exposure to the heavy metal, lead, has been shown to inhibit drug metabolism; whereas intensive exposure to chlorinated insecticides, and other halogenated hydrocarbons such as polychlorinated biphenyls, has been shown to enhance the metabolism of test drugs such as antipyrine and phenylbutazone. An intentional source of exposure to foreign chemicals is cigarette smoke. Cigarette smoke contains polycyclic hydrocarbons, which are known inducers of hepatic mixed function oxidases. A number of studies have shown that cigarette smoking can alter the pharmacological action and/or the metabolism of some drugs. Pharmacokinetic studies have shown that cigarette smoking decreases the bioavailability of phenacetin and increases dosage requirements of theophylline by enhancing their rate of metabolism. Data, which are not very conclusive, indicate that heavy marijuana use may have an inhibitory effect on metabolism of some drugs and an inducing effect on others such as theophylline. Dietary factors may also play a significant role in the regulation of drug metabolism. Charcoal broiling which introduces polycyclic hydrocarbons into foods has been shown to enhance the metabolism of the test drug, antipyrine, and of such commonly used drugs as phenacetin and theophylline. Such intentional or unintentional exposure to environmental chemicals which may alter the rates of drug metabolism in man indicates the importance of individualisation of drug therapy.