1. Ten compounds from the Merck Research Laboratories pipeline were selected to evaluate the utility of using intrinsic clearance derived from recombinantly expressed cytochromes P450 (CYP) and physiologically based pharmacokinetic modelling to predict Phase I pharmacokinetics using simCYP®. The compounds selected were anticipated to be eliminated predominantly by P450 metabolism. 2. There was a reasonable agreement between the predicted and actual clinical exposure with 80% of the predicted exposures being within three-fold of the observed values. Furthermore,prediction of Ct (plasma concentration at a specified time point) and Tmax were acceptable with greater than or equal to 70% of the predicted data being within three-fold of the observed values. However, prediction of Cmax was unreliable and may have been due to error in predicting the time-dependent change in volume of distribution and/or error in estimating absorption rate. 3. Although it is acknowledged that research is needed to improve predictive performance, the data presented are supportive of using recombinant P450 intrinsic clearance and physiologically basedpharmacokinetic modelling to predict Phase I pharmacokinetics for compounds eliminated by P450 metabolism.