Dependent pooling occurs in postural orthostatic tachycardia syndrome (POTS) related to defective vasoconstriction. Increased venous pressure (Pv) >20 mmHg occurs in some patients (high Pv) but not others (normal Pv). We compared 22 patients, aged 12-18 yr, with 13 normal controls. Continuous blood pressure and strain-gauge plethysmography were used to measure supine forearm and calf blood flow, resistance, venous compliance, and microvascular filtration, and blood flow and swelling during 70 degrees head-up tilt. Supine, high Pv had normal resistance in arms (26 +/- 2 mmHg x ml(-1) x 100 ml x min) and legs (34 +/- 3 mmHg x ml(-1) x 100 ml x min) but low leg blood flow (1.5 +/- 0.4 ml x 100 ml(-1) x min(-1)). Supine leg Pv (30 +/- 2 vs. 13 +/- 1 mmHg in control) exceeded the threshold for edema (isovolumetric pressure = 19 +/- 3 mmHg). Supine, normal Pv had high blood flow in arms (4.1 +/- 0.2 vs. 3.5 +/- 0.2 ml x 100 ml(-1) x min(-1) in control) and legs (3.8 +/- 0.4 vs. 2.7 +/- 0.3 ml x 100 ml(-1) x min(-1) in control) with low resistance. With tilt, calf blood flow increased steadily in POTS with high Pv and transiently increased in normal Pv. Calf volume increased in all POTS patients. Arm blood flow increased in normal Pv only with forearm maintained at heart level. These data suggest that there are (at least) two subgroups of POTS characterized by high Pv and low flow or normal Pv and high flow. These may correspond to abnormalities in local or baroreceptor-mediated vasoconstriction, respectively.