1. 1. Cardiovascular adjustments to voluntary exercise and diving have been studied in two species of penguins ( Pygoscelis papua and P. adeliae) using techniques for telemetry of blood flow, blood pressure and heart rate. 2. 2. Free exercise resulted in a rise in heart rate (from 122 to 278 beats/min), mean blood pressure (from 92 to 163 mm Hg) and femoral blood flow (from 24 to 59 ml/min). This response was little modified when birds were exercised on a treadmill. 3. 3. Arterial oxygen tension during exercise rose from 70 to 80 mm Hg while venous oxygen tension fell from 38 to 28 mm Hg, resulting in increased oxygen utilization. Severe exercise reduced arterial pH from 7·55 to 7·25. 4. 4. In free swimming, heart rate was 227 beats/min, femoral flow, 40 ml/min and carotid flow, 55 ml/min. During voluntary dives heart rate fell to approximately one-third as did femoral flow to one-fourth and carotid flow by 30 per cent. Forced dives differed by causing considerable struggling accompanied by transient changes in heart rate and blood flows. 5. 5. Only minor changes in blood oxygen tensions and pH were observed after free diving. Forced dives of 90–120 sec caused reductions of arterial oxygen tension from 82·4 to 33·5 mm Hg, venous oxygen tension from 43·5 to 27·5 mm Hg while pH fell from 7·48 to 7·37. Recovery showed a continued but transient decline in pH and venous oxygen tension. At 30 sec pH was 7·27 and venous pO 2, 25 mm Hg, while arterial oxygen tension had risen near control, 71·5 mm Hg.