This study is a breath-by-breath analysis of the effects of 5, 10, and 15 cmH2O positive-pressure breathing (PPB) on man's steady-state breathing pattern. Inspiratory (TI), expiratory (TE), and cycle (TT) durations, tidal volumes (VT), minute ventilation (VE), mean inspiratory flow rate (VT/TI), and mean expiratory flow rate (VT/TE) were determined from pneumotachograph and Wedge spirometer recordings before and during steady states on PPB. End-tidal CO2 was continuously recorded. Seventeen adults, seated in a full body-box, breathed quietly for 8 min through a mouthpiece on a bag-in-box. Pressure in the body-box was lowered to the desired level prior to 4 min of stress. On all pressure levels, end-expiratory volume, VT, VE, VT/TI, and VT/TE increased; end-tidal CO2, TE, and TT decreased with no consistent change in TI. Calculated alveolar ventilations indicated that the increases in VE were true hyperventilations. Each individual increased VE by using a unique combination of VT, TI, and TE. End-expiratory volume increased less and expiratory flow increased more than would occur passively. Hence, it is concluded that active reflexes account for the resistance of the systems to the passive distention, the facilitation of expiratory flow, and the shortening of TE.