Hypertension after cardiac surgery is common and requires accurate control by carefully regulated infusions of drugs such as sodium nitroprusside. A microprocessor-based controller has been designed to close the loop between the blood pressure response and the infusion rate of a hypotensive drug. This system has been refined by computer simulation of the blood pressure response to sodium nitroprusside and by experience gained in using the controller in eleven patients in the early recovery period after cardiac surgery. The controller was able automatically to maintain blood pressure within 10% of a specified value. Provision of sophisticated safety features in automatic drug infusion controllers is essential for patient protection.