Preliminary to assigning significance to therapeutically induced changes in plasma colloid osmotic pressure (COP), it is necessary to know how this parameter varies under controlled conditions such as absolute bed rest. Accordingly, the following data were obtained at eight-hour intervals from seven healthy subjects maintained supine for two separate 48-hour periods: plasma COP, alveolar-arterial oxygen pressure (Po2) gradients, pulmonary shunt fractions, cardiac index, total peripheral resistance, and mean arterial blood pressure. Results indicated that with ambulation, oncotic pressure rose significantly. With recumbency, plasma COP varied considerably (± 10%), while the other data reflecting cardiopulmonary function remained normal. The extent of this variation of plasma COP in recumbent normal humans was the same range as the changes that others report effected by albumin and diuretic therapy.