1. The relationships between the depth of a breath and the durations of the inspiratory and expiratory phases have been studied in cat and in man during rebreathing, and in cat using artificial inflations of different magnitudes and timings.2. In the cat, the apparent volume threshold for termination of inspiration (Hering-Breuer threshold) decreased with time from the onset of the inspiratory phase.3. Both in rebreathing experiments and with artificially imposed inflations in the cat, the inspiratory duration T(I) was dependent upon tidal volume V(T), and this dependence could be expressed by a hyperbolic relationship of the form (V(T)-V(0)) T(I) = C where V(0) and C are constants.4. The time course of this ;Hering-Breuer' threshold was dependent on intact vagus nerves. After vagotomy the inspiratory duration remained essentially constant with changes in tidal volume produced either by artificial inflation or by the increased respiratory drive due to accumulation of CO(2) during rebreathing.5. In man during rebreathing, the relation between volume and inspiratory duration typically showed two different characteristics. 1, at tidal volumes up to 1.5-2 times eupnoeic values, inspiratory duration did not change as tidal volume increased in response to increased P(CO2). This range of operation has been designated range 1. 2, as tidal volume increased above this range 1 a second range designated range 2 was observed where inspiratory duration was volume dependent in the same manner as in the cat.6. In cat under pentobarbitone anaesthesia, a range 1 operation was not seen except after vagotomy. However, under urethane anaesthesia a range 1 plus a range 2 operation could be seen.7. The differences between cat and man appeared to be largely quantitative rather than qualitative.8. In both cat and man, expiratory duration was dependent on inspiratory duration, usually with a linear relationship.9. The experimental results were assembled in the form of an inspiratory characteristic and a timing relationship that serve as a model of the respiratory mechanisms controlling the depth and rate of breathing. The model predicts that the depth and duration of a breath are related in a definite manner fixed by the system characteristics and that ventilation is adjusted by setting the appropriate operating point on these characteristics. The operating point is determined primarily by how quickly lung volume increases, i.e. the rate of increase of inspiratory motor activity.