Fourteen dogs were anesthetized and rendered hypoxic by ventilation with 8% oxygen. Efficacy and toxic effects of a small (75-ml prime) membrane oxygenator was evaluated during a 24-hour period of perfusion. Except for a transient thrombocytopenia, there was no significant hematologic toxic changes that could be ascribed to the oxygenator. The animals demonstrated no toxic effects in the six-day observation period following perfusion. This small oxygenator transfers 18 to 23 ml of oxygen per minute at an average blood flow of 450 ml/min over a 24-hour period. Its efficiency and relative lack of toxic effects to blood elements provide a sound experimental basis for its further exploration on a clinical basis where prolonged partial respiratory assistance is required.