In healthy men and women, increases in muscle sympathetic nerve activity, circulating catecholamines and total peripheral resistance during upright tilt are similar (Fu et al. 2005). Reflex vasoconstriction when upright protects against excessive lower body pooling, and decreases in ultrasound femoral artery blood flow during tilt (~ 40%) correlate with those of plethysmographic lower limb blood flow (~46%) in healthy subjects of mixed gender (Kooijman et al. 2007). We used ultrasound to investigate whether tilt responses of the popliteal artery and long saphenous vein are also the same in men and women. Young healthy subjects age 18-23 years, 14 men and 15 women (tested in their menstrual phase) participated in a study approved by the University of Birmingham Local Research Ethics Subcommittee. Mean arterial pressure (MAP) and heart rate (HR, Portapres), calf volume (strain gauge plethysmography), popliteal artery diameter and velocity and long saphenous (LS) vein diameter (ultrasound) were measured during supine rest and either 5 min of non weight-bearing 60^o head up tilt (HUT) or 5 min supine venous distension (thigh cuff inflated to 50 mmHg). Responses during tilt or venous distension were compared to supine rest by repeated measures Anova with gender and intervention as factors. During tilt, MAP did not alter in either group while men showed bigger increases in HR than women (17 ± 3 beats.min^-1 v. 9 ± 2, mean ± S.E.M., p<0.01). During venous distension, neither MAP nor HR altered. Calf volume increased to a similar extent in men and women during venous distension (2.5 ± 0.3 %, 2.5 ± 0.2 %, ns) but during tilt, calf volume increased significantly more in men than it did during distension (4.0 ± 0.2 %, p<0.001) while in women, tilt-induced increases in calf volume were not different (2.6 ± 0.3 %). Conductance of the popliteal artery was calculated from flow (cross sectional area x velocity) and mean arterial pressure (corrected for venous distension pressure, and for height difference during tilt). Popliteal conductance did not change significantly during tilt (men +6 %, women +9 %) or distension (men +15 %, women +8 %). LS vein diameter increased 70-80% during tilt and was greater in men than women (3.5 ± 0.3 mm v 2.3 ± 0.2, p<0.01). It increased 50-60% during distension and was similar in size in the groups (men 2.5 ± 0.2 mm v. 2.1 ± 0.2, ns). We conclude that, unlike the femoral artery, the popliteal artery does not show postural vasoconstriction. The larger increase in calf volume in men than women during tilt despite similar estimated hydrostatic pressures at the level of the strain gauge (men 69 ± 4 mmHg, women 65 ± 2) is associated with greater vein distensibility. This may be related to different gender proportions of lean and fat tissues in the lower leg segment.