An outline of the pharmacology of important components of extemporaneous prescriptions will be undertaken over four issues of the journal. Substances such as dithranol, salicylic acid and coal tar will be examined in some depth with regard to their mode of action, uses and side effects. In addition, examples of extemporaneous formulations containing these substances will be provided along with advice regarding their use. Substances such as sulphur, calamine, podophyllin and menthol will also be reviewed using the same format but in less detail. The articles are designed to provide practical knowledge in the area of extemporaneous prescribing. Complicated areas of pharmaceutical chemistry, unless relevant to the clinical setting, will not be discussed.