It has previously been suggested that subjects who are initially slow metabolisers of drugs have a greater potential for induction of their drug metabolising enzymes than subjects with initial high rates of metabolism. This inference is based on observations made of changes in half-life of antipyrine. The purpose of this presentation is to reanalyse data previously presented in the literature with reference to clearance, a more precise estimate of drug metabolising activity, and half-life a parameter derived from both clearance and distribution.In one study in non-obese subjects, approximately 80% of intersubject variation in the change of total antipyrine clearance can be explained by differences in body size, particularly differences in liver volume. Furthermore, the relationship between initial antipyrine half-life and the percentage change in antipyrine half-life following induction can be explained by the association between each parameter and body weight. These observations imply that the potential for enzyme induction, and therefore for drug interactions based on enzyme induction, is present in all subjects and that intersubject variance in steady-state drug concentrations are as wide following induction as before induction.