The goal of this study was to develop a human physiologically based pharmacokinetic (PBPK) model for the chemical HCFC-123 (2,2-dichloro-l,l,l-trifluoroethane) and its major metabolite, trifluoroacetic acid (TFA). No human kinetic data for HCFC-123 are available, thus a corollary approach was developed. HCFC-123 is a structural analog of the common anesthetic agent halothane (2-bromo-2-chloro-l,l,l-trifluoroethane) and follows a common pathway of oxidative biotransformation, resulting in the formation of the same metabolite, TFA. In this study, halothane models for rats and humans were developed and validated. Then the corollary approach was used to develop a human HCFC-123 model from a rat HCFC-123 model. This strategy was implemented by using a previously validated PBPK model for HCFC-123/TFA in the Fischer 344 rat as a template model for halothane in rats. Model predictions were then compared to, and were in good agreement with, measured values for the concentration of halothane in rat blood and fat tissue. A human PBPK model for halothane was developed. The identical model structure (with the exception of the description for the fat compartment) that was used to describe halothane and TFA in the rat was used for describing halothane and TFA in the human. Human physiological parameters for tissue volumes and flows were taken from the literature, and human tissue partition coefficients for halothane were measured in the laboratory. Based on reported similarity in metabolism of halothane by humans and rats, metabolic constants for halothane in the rat were used in the human model, and specific parameters describing the kinetics of TFA were estimated by optimization. The model was validated against human exposure data for halothane from six published studies (expired breath concentrations of halothane and serum/urine data for TFA). A similar approach was then used to derive a human HCFC-123 model for humans from the HCFC-123 rat model. The corollary approach described here illustrates the innovative use of template model ' To whom correspondence should be addressed. structures to aid in the development and validation of models for structural analogs with similar metabolism and activity in biologic systems. Furthermore, given that the PBPK model adequately describes the kinetics of halothane in rats and humans and of HCFC-123 in rats, use of the human PBPK model is proposed for deriving dose-response estimates of human health risks in the absence of human kinetic data, c IWS society of Toricofcpgjr