Although volume, pressure or flow in superficial veins have been studied extensively, little is known about venous blood velocity during thermal stress. Most authors have suggested that the velocity is decreased in the dilated superficial veins during heat stress to facilitate heat loss, and is increased during cooling as the vein is constricted. Duplex ultrasound has been used to study saphenous cross sectional area (CSA) and mean maximal venous blood velocity (BV) in ten healthy volunteers (age 22-31 years). Compared with unstressed mean values, 4.8 (SD 2.6) mm2, CSA increased to 9.3 (SD 2.1) mm2 (P < 0.005) during heat stress and decreased to 2.1 (SD 1.9) mm2 (P < 0.005) during cold stress. These results are consistent with previous studies, but the absolute CSA of the saphenous vein has never been estimated during thermal stress. The BV increased from 0.07 (SD 0.02) m.s-1 to 0.29 (SD 0.11) m.s-1 (P < 0.005) during warming. During cooling, BV tended to decrease: 0.05 (SD 0.03) m.s-1 (N.S). We would suggest that heat loss during thermal stress can be facilitated by the rapid turnover of warm blood, and not (as usually suggested) by the prolonged cooling of each blood sample in the dilated superficial veins.