Numerous studies have focused on alterations in plasma volume (PV) on interventions like quiet standing, exercise, or heat stress. However, no method seems capable of truly estimating the PV alteration. Therefore, an attempt was made to validate commonly used indexes of PV changes. Quiet standing was used to cause graded PV reductions estimated from hemoglobin and hematocrit (Hb/Hct) and from serum concentrations of total protein, albumin, and "large proteins" (LP; total protein minus albumin). Results indicated the following. 1) Hb/Hct, with the merit that the marker (erythrocyte) stays within the circulation, reflect accurately a small-to-moderate PV loss (</=10% of control PV). At large PV decrease (>/=15-20% of control), however, F-cell shift can cause Hb/Hct to underestimate the response by up to 25-30%. 2) Albumin and total protein underrate PV loss due to protein escape (mainly albumin) from the circulation. 3) LP also underestimate the PV decline due to protein escape but can often predict large PV reductions clearly better than Hb/Hct. 4) Prolonged standing can lead to pronounced 25% PV decline.