The purpose of this study was to investigate whether ambient temperature influences both the rate of leg swelling during orthostasis and the oedema-preventing effect of the skeletal muscle pump. Using mercuryin-rubber strain gauges, volume changes were measured in the calf (n = 34) and near the ankle (n = 24) in healthy volunteers aged 19–33 years. Measurements were performed during 12 min of motionless standing in an upright posture and during 17 min of cycle exercise at intensities of 50 W and a pedalling rate of 50 rpm. The experiments were done in an air-conditioned chamber at temperatures of 20, 28 and 36°C and 50% relative humidity. The rate of leg swelling, which occurred while standing, did not differ significantly among the three temperatures. The mean increases in calf volume during 10 min (min 2–12) orthostasis were 1.6 (SEM 0.1)%, 1.9 (SEM 0.2)% and 2.0 (SEM 0.2)% at 20, 28 and 36°C respectively. In the ankle region the mean values were 0.9 (SEM 0.1)%, 1.0 (SEM 0.1)%, and 1.0 (SEM 0.1)% at the three temperatures, respectively. Exercising at low temperatures continuously reduced the volume of the leg, but at 36°C the leg volume did not change significantly either at the calf or near the ankle. The mean volume changes measured beteeen min 2 and min 15 were, at the calf, −1.1 (SEM 0.1)%, −0.8 (SEM 0.2)%, and −0.02 (SEM 0.1)% at 20, 28 and 36°C, respectively. Near the ankle the mean changes were −0.7 (SEM 0.1)%, −0.3 (SEM 0.1)%, and +0.2 (SEM 0.1)%. A mathematical description of the time course of the changes supported the view that the changes in the leg volume, which occurred after a subject had been tilted from a supine to an upright posture, consisted of two distinct components. First, the capacitance vessels filled in an exponential manner and second, the leg volume increased linearly, due to augmented transcapillary filtration.