Some ventilatory and circulatory parameters were studied in 17 very obese patients before and after weight reduction following jejunoileal bypass. A low vital capacity and signs of impaired lung function with intrapulmonary shunting, increased alveolar-arterial Po2 difference and low Pao2 were found. Although the spirometric values improved significantly after weight reduction, the ventilatory disturbance persisted. A normal response to inhalation of CO2 was seen. The total blood volume was high and did not change after weight reduction. However, if calculated as blood volume per kg body weight, the values were lower than normal, and they increased as a consequence of weight reduction. Cardiac output was slightly lower than normal in relation to oxygen consumption. Total peripheral resistance was normal. Arterial blood pressure, which was in the high normal range preoperatively, decreased significantly after weight reduction. Total doses of intravenous anaesthetic agents and muscle relaxants were the same as for patients of normal weight. The importance of preoperative evaluation and of respiratory care of obese patients undergoing elective surgery is stressed.