ABSTRACTAn on‐line minicomputer‐based system for determining breath‐by‐breath respiratory gas exchange (oxygen consumption and carbon dioxide production), ventilation (tidal volume, respiratory period, and minute ventilation), partial pressures of end‐tidal oxygen and carbon dioxide, and heart rate is described. The system software includes routines for calibration, signal acquisition and processing, and disk storage of breath‐by‐breath data. The software is written in Fortran IV and runs under Digital Equipment Corporation's RT‐11 system monitor. On‐line (real time) computations of breath‐by‐breath data include instantaneous corrections of: 1) expired gas flow for alterations in gas viscosity, and 2) expired gas fractions for gas analyzer response characteristics. Additional programming provides rapid data reduction and averaging over a user‐selected time base. Twelve healthy male volunteers were monitored at rest and during 10‐min bouts of exercise at 300, 600, and 900 kpm · min‐1, and during recovery. Both the transient and steady state alterations in cardiopulmonary response to these exercise tests were found to compare favorably with standard values established in the exercise physiology literature. The sensitivity and accuracy of this method to monitor both phasic and tonic alterations in cardiopulmonary function during stress underscores its suitability for a wide range of ps