1. UK-453,061 is a novel second-generation non-nucleoside reverse transcriptase inhibitor (NNRTI). Following intravenous bolus administration of UK-453,061 in male rat and infusion administration in dog, UK-453,061 had the following mean pharmacokinetic properties: elimination T(1/2) of 1.6 and 2.4 h, CL(p) of 26 and 10 ml min(-1) kg(-1) and V(ss) of 1.6 and 2 l kg(-1), respectively. 2. The half-lives of UK-453,061 disappearance in recombinant human CYPs 2C8, 2C9, 2A6, 2E1, 1A2, 2C19, 2D6 and 3A4 were 71, 100, 56, 101, 61, 34, 60 and 8 min, respectively. The disappearance half-life of UK-453,061 in human liver microsomes in the presence of UDPGA was 90 min. 3. Human clearance values were predicted using single-species scaling from in vivo data and from in vitro data using SimCYP. The human distribution of UK-453,061 was estimated using an in silico physiologically based pharmacokinetics (PBPK) methodology and absorption was predicted from measured physicochemical, permeability, and solubility data using GastroPlus and SimCYP. The C(max) was predicted to be 68, 185, 149% of the actual mean value using rat, dog and in vitro predicted values of human clearance at 30 mg and 53, 150, 29% of actual at 500 mg. The area under the curve (AUC) was predicted to be 73, 285 and 142% of the actual mean value using rat, dog and in vitro predicted values of human clearance at 30 mg and 52, 212 and 35% of actual at 500 mg. 4. This study demonstrates the utility of using in silico PBPK approaches to make predictions of human pharmacokinetics before dosing for the first time in humans.