The mechanical responses (active and resting tension, dP/dt, TPT) and ionic exchange characteristics (Ca(++), K(+), Na(+)) which follow upon a variation in temperature, rate, and [K(+)](o) were studied in the rabbit papillary muscle and arterially perfused rabbit interventricular setpum. Abrupt changes in temperature provided a means of separating the contributions of rate of development (intensity) of active state and duration of active state to total active tension development (approximated by isometric tension). Threefold changes in duration of active state with proportional changes in active tension can be induced without evidence for alteration of Ca(++), K(+), or Na(+) exchange. Abrupt cooling produced a moderate ( approximately 15%) increase of dP/dt which suggests an augmentation of active state intensity. Evidence is presented to suggest that this increase of dP/dt is based upon an increase in membrane Ca(++) concentration which occurs secondary to inhibition of active Na(+) transport. The alterations in ionic exchange and active state produced by variation of temperature are discussed in terms of a five-component control system.