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Changes in cardiopulmonary function in Beagle dogs were studied during two methods of lung lavage and during unilateral hypoxia. In one method of lavage, dogs were hyperventilated and saline was instilled relatively rapidly through a single-lumen tracheal tube. In another lavage method, a double-lumen tube separated right and left bronchi and saline was instilled more slowly into the dog's right lung. Unilateral hypoxia was produced by directing right lung airflow to a rebreathing bag. Few blood gas or venous admixture changes occurred during single-lumen lavage, but blood oxygen decreased and venous admixture increased during both double-lumen lavage and unilateral hypoxia. Heart rate, cardiac output, and systemic arterial pressure decreased during single-lumen lavage, but not during double-lumen lavage or unilateral hypoxia. Apparently, most alveoli were gas filled during single-lumen lavage and unilateral hypoxia was not imposed. Similar changes during double-lumen lavage and unilateral hypoxia suggested that unilateral hypoxia was a major change-producing factor during double-lumen lavage.