1. In both Pseudemys and Testudo, blood pressures are identical in all chambers of the ventricle. Systolic pressures are not measurably different in the ventricle and systemic arteries but are 0-5-2-0 cm H2O lower in the pulmonary artery due to the resistance of the pulmonary outflow tract. Diastolic pressures are the same in all systemic arteries but are substantially lower in the pulmonary artery. It is concluded that the systemic and pulmonary circulations are perfused by a single pump during both apnoea and lung ventilation. 2. Flow profiles in pulmonary and systemic arteries are characteristicallly different. Substantial changes in cardiac output may occur during intermittent breathing and apnoea, especially if there are large fluctuations in heart rate. Pulmonary flow increases during lung ventilation due to vasodilation of the lung vasculature. Systemic is also affected though the increase is usually smaller. 3. Any separation in the blood pumped by the single ventricle must be maintained by laminar flow patterns and the composition of the blood in each of the major arteries should reflect their relationship to these patterns.