The need for closed-loop control of blood pressure in acute hypertension after cardiac surgery was evaluated by estimating the amount of nurse-time required to adjust the infusion rate of sodium nitroprusside. The evaluation consisted of a nationwide telephone survey of head nurses in postsurgical ICUs at 9 institutions performing cardiac surgery. By nurse estimate, the average time spent adjusting the nitroprusside infusion rate was 26% of the time the patients were being infused. When calculated from a hierarchical list of questions, the average nitroprusside adjustment time was 16%. Assuming that a closed-loop controller would be accepted in these ICUs, they would be used 40% of the time that the nurse and patient are on a 1-to-1 basis. It was concluded that a technically acceptable control design exists for clinical use, that at the present time there is a need for such a controller, and that the controller would be accepted by the clinical staff.