A model for the conversion of respiratory neural output to mechanical output, and vice versa, is described. Neural output was expressed in terms of isometric pressure generated at passive functional residual capacity. The mechanical response time constant of respiratory muscle was assumed to be 0.06 s. The effect of volume and configuration on pressure output was modeled after the data of Grassino et al. (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 44: 829-839, 1978). Equations were developed to examine the effect of different configurational pathways during inspiration. We utilized Hill's hyperbolic force-velocity relation to model the effect of flow on pressure output. The pressure asymptote of the hyperbola was considered to be similar to that in other skeletal muscles (0.25 isometric pressure). The flow asymptote was derived from data obtained during maximal voluntary inspiration. A major feature of the model is the dependence of volume, configuration, and flow-related pressure losses on level of inspiratory activity. The practical effect of potential errors and the overall accuracy of the model are presented in the two succeeding communications.