We consider how the optimal selection of the profile of imposed work rate, coupled with rigorous, statistically justified analysis of the pattern of the pulmonary gas exchange response, makes it possible to assemble a control model incorporating the proportional role of the muscle, circulation, and gas stores. Gains, time constants, and delays may be assigned to the components of the response and its linearity assessed. These techniques also allow the investigator to examine the features of poorly understood and even unexpected response patterns. Recent interest in the analysis of the non-steady state of exercise-in normal subjects and in patients with gas exchange defects-has led to an improved understanding of the sub-threshold dynamics. At work rates above the lactate threshold, the more complex kinetics are to date poorly described, and hence poorly understood, remaining a fertile area for the application of control-systems techniques to exercise.