A transient thermal model of the portable life support system (PLSS) is being developed for use in thermal control studies. The transient thermal PLSS (TTPLSS) model has been developed and implemented using SIMULINK in conjunction with MATLAB. The TTPLSS has been developed with modularity and flexibility in mind so that alternative PLSS designs and configurations can easily be implemented and evaluated. The basic structure and functionality of the TTPLSS SIMULINK model is described and demonstrated. The various thermal dynamics issues associated with the PLSS such as time delays and the dynamics of individual components are discussed and considered.