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Body position exerts considerable influence on transcapillary balance of protein-poor fluid, i.e., on the whole-body Starling equilibrium. Movement into upright position causes hemoconcentration whereas supine position makes hemodilution. This means that the alteration of blood and plasma volume influences, e.g., plasma protein concentration, hematocrit, and blood hemoglobin content. The application of the mechanical oscillator technique for high-precision density measurements on capillary blood and plasma, especially to quantify the time-course of alterations caused by posture, is discussed in this paper. Tilting into upright position (70 degrees) 45 min after recumbency led to an average increase of 6.6% after 10, and of 11.1% of plasma volume after 30 min in 12 test persons. In 13 test persons, tilting back into supine position (0 degrees) after 30-60 min of standing (70 degrees) caused a mean increase of plasma volume amounting 6.5% after 10, and of 10.5% after 30 min. Postural blood density variations showed shapes similar to that of plasma density, indicating blood volume alterations ranging between 5 and 10%. It is emphasized that, as a consequence for clinical practice, the body positioning before and during blood sampling must be taken into consideration especially in case of precise controls of the course of hematological variables, and in case of statistical comparisons among several test groups.