A study was conducted to determine non-invasively the effects of endurance training on the size of the inferior vena cava in humans. Twelve healthy male subjects were assigned to either an exercise-trained group (ET, n=7) or a sedentary control group (S, n=5) . The ET group underwent cycle-endurance training for 8 weeks (80%Vo₂max, 40 min/day, 4 days/week) . The S group led normal lives during the 8-week period. Before and after the training period, cross-sectional areas (CSA) of the inferior vena cava and the ascending and abdominal aorta were measured by echography. The CSA of the inferior vena cava after training was significantly larger than that before training in the ET group. There was no significant difference in the S group. These results indicate that the inferior versa cava can be morphologically altered as an adaptive response to endurance training. We consider that this adaptation partly contributes to the improvement in the efficiency of venous return from exercising muscles to the heart. Although the present training also increased the CSA of the aorta, the degree of change was smaller than that seen in the inferior vena cava, implying that the factors of adaptation and adaptability to endurance training in the inferior vena cava differ from those in the aorta.