Inter-compartmental body-fluid distribution is contingent upon posture, exercise state and environmental temperature. This investigation aimed at quantifying the distribution of intra- and extravascular fluid volumes during postural manipulations. Fluid shifts were measured in eight males utilizing a simultaneous, radionuclide dilution technique, in which radioiodinated serum fibrinogen, radiochromated erythrocytes, radiobromine and tritiated water were used to measure plasma, red cell, extracellular and total body water volumes. Subjects were exposed to three postural changes [seated (control), supine and standing] for 30 min at an air temperature of 22.0 degrees C, with each posture separated by 30 min seated rest. Total body water content remained stable throughout postural changes (P = 0.842). Relative to seated volumes, BV increased by 89 mL when supine, and decreased by 406 mL while standing (P = 0.003), with such shifts being primarily a result of plasma movement (P = 0.011). Red cell volume changes were not significant. Vascular fluid lost during standing was filtered into the interstitial compartment (P = 0.014), with the extracellular and intracellular volumes remaining unaffected. (P = 0.271 and P = 0.800, respectively). These observations confirmed the influence of posture on inter-compartmental body-fluid distribution. The intravascular fluid loss when standing was caused by the filtration of plasma into the interstitium, while, during supine rest, intravascular volume increased, reflecting fluid flux from the interstitium to the circulation.