This paper presents preliminary results for using dynamic systems models to describe physiological and behavioral responses (cortisol and activity) to emotionally stressful events. Linear discrete-time models are used to approximate the nonlinear model of the LHPA axis around an assumed nominal operating condition. Measurements are taken of cortisol (from saliva) and activity (with an accelerometer). These two measurements are considered as either inputs or outputs of the model. Modeling choices are discussed in detail. Results are presented that indicate activity is better interpreted as an input and cortisol as an output. In addition, the paper discusses briefly how the resulting dynamic systems models can be used for statistical analysis, as well as for integrating across multiple levels of stress responses.