The effects of trimetaphan camsylate and sodium nitroprusside on the catecholamine response to cardiac surgery were compared in a randomised double blind study of twelve male patients undergoing elective myocardial revascularisation. The solutions were titrated to maintain a mean arterial pressure of 70-85 mm Hg before and after bypass and less than 70 mm Hg during bypass. The rise in plasma adrenaline during cardiopulmonary bypass in the sodium nitroprusside group was significantly greater than that in the trimetaphan camsylate group. There was a smaller rise in plasma noradrenaline in the sodium nitroprusside patients but this was significantly higher than in the patients receiving trimetaphan camsylate. Administration of trimetaphan camsylate provides a simple and effective way to reduce catecholamine release during cardiopulmonary bypass.