Cardiorespiratory effects of ketamine and Althesin were measured in two groups of premedicated patients with cardiac disease. The drugs were given in clinically equivalent doses with a second dose administered about 10 min after induction. The first dose of ketamine caused a marked increase in systemic and pulmonary arterial pressure, heart rate, and central venous and wedge pressures and cardiac index. The first dose of Althesin caused a decrease in systemic arterial pressure, central venous pressure, cardiac index and heart work, but little change in heart rate. The second dose of induction agent was administered before the cardiorespiratory effects of the initial dose had resolved. The second dose of Althesin caused changes similar to those following the first dose, but less marked. The changes following the second dose of ketamine were opposite to those following the first dose.