The purpose of this study is to develop a new method for the measurement in humans of the compliance of the microvascular superficial venous system of the lower limb by near-infrared spectroscopy (NIRS). This method is complementary to strain-gauge plethysmography, which does not allow compliance between deep and superficial venous or between venous and arterial compartments to be distinguished. In practice, hydrostatic pressure (P) changes were induced in a calf region of interest by head-up tilt of the subject from alpha = -10 to 75 degrees. For P < or = 24 mmHg, the measured compliance [0.086 +/- 0.005 (SD) ml. l(-1). mmHg(-1)] based on NIRS data of total, deoxygenated, and oxygenated hemoglobin, reflects essentially that of the superficial venous system. For P > or = 24 mmHg, no distinction can be made between arterial and venous volumes changes. However, by following the changes in oxy- and deoxyhemoglobin in the P range -16 to 100 mmHg, it appears to be possible to assess the characteristics of the vasomotor response of the arteriolar system.