The hypoxemia, elevated vascular pressures, and cardiac arrhythmias occurring during endotracheal suction may be related both to suction and the interruption of ventilation during suction. Although effects of suction vs. apnea have been compared in healthy patients, interruption of ventilation for purely investigational purposes precludes such a study in critically ill patients. Thus, in the present study, cardiovascular and blood gas changes attendant to endotracheal suction or equivalent periods of apnea were compared in anesthetized, paralyzed dogs in acute respiratory failure induced by oleic acid. Suction of 45-sec duration during interruption of intermittent positive pressure ventilation (IPPV) was associated with decreases in Pao2, pH, and heart rate and increases in Paco2, cardiac output, pulmonary arterial and systemic arterial pressure. These changes were not different from those observed during interruption of ventilation (apnea) alone. Cardiovascular and blood gas changes were also similar when suction and apnea were compared during interruption of continuous positive pressure ventilation (CPPV). Neither apnea nor suction was associated with cardiac arrhythmias.