PIP: A comparative study in the same patients of the 2 methods for determining the amount of blood needed to obtain optimal blood-volume was conducted. The central venous pressure (CVP) was monitored continuously by an intravenous catheter with its tip in the superior vena cava and connected to a saline-solution manometer with the zero point at 1/2 the anteroposterior diameter of the chest. The measured normal central venous pressure ranged from 2-10 cm of water. The blood-volume was measured with a semi-automatic instrument with which accurate serial determinations of blood-volume were obtained. When the normal blood-volume was not known by direct measurement, it was calculated as the product of the patient's weight in kg times 65, 70, or 75 ml, depending on the body habitus and nutritional state of the patient. 6 case reports, culled from a study of over 80 patients in shock, in whom these 2 measurements were taken serially over a period of many hours, are reported. They were selected in order to illustrate the variety of problems encountered in restoring blood-loss. Direct measurement of the blood-volume with the aid of the modern computer devices yields reliable results even in advanced shock and facilitates precise correlation of the deficit, yet in patients with or without cardiac disability, and particularly those of middle or advanced age, monitoring of the CVP combined with serial blood-volume determinations provides the best guide.