The concept of microdosing has been around for approximately 10 years. In this time there have been an increasing number of drugs reported in the literature where the pharmacokinetics at a microdose have been compared with those observed at a therapeutic dose. Currently, approximately 80% of the microdose pharmacokinetics available in the public domain have been shown to scale to those observed at a therapeutic dose, within a twofold difference. Microdosing is now being extended into areas of drug development other than purely pharmacokinetic prediction. Microdosing has been applied to the study of drug-drug interactions by giving human volunteers a microdose of the candidate drug before and after the administration of a drug known to inhibit or induce certain enzymes, such as the cytochrome P450s. Early data on the metabolism of a drug candidate can be obtained by administering a (14)C-drug to human volunteers and comparing the plasma concentration-time curves for total (14)C and unchanged parent compound. Full metabolic profiles can be generated as an early indication of the drug's metabolism in humans, prior to Phase 1 clinical studies. Microdosing is also being applied to situations where the concentration of a drug in cell or tissue types is key to its efficacy. The application of microdosing as a tool in drug development is therefore widening into new and previously unforeseen fields.