BACKGROUND: Many infusion devices have been used in anesthetic practice, some of them are balloon type infusor consisted of rubber or silicone materials. This study was done on the basis of that the flow rate will vary according to the volume of balloon type infusor because the shape and elasticity of the balloon is changed. METHODS: We examed infusor-A consisted of silicone (n=15) and Infusor-B consisted of rubber (n=15). Each model was 2 ml/h, total volume 100 ml type. The infusor was filled with normal saline 100 ml and the inside pressure of the infusor was measured according to the reservoir volume change. Infusion rate was calculated by dividing the infused volume by infused time. The infused volume was measured by the weight reduction of the reservoir. RESULTS: The monitored inside pressure of the reservoir was divided into 3 ranges; the range of the pressure decreasing (100-85 ml), maintaining (85-8 ml), and increasing (8-0 ml) with the infusor volume. The flow rate varied with the infusor volume; in the large (100-85 ml) and small (8-0 ml) infusor volume, the infusion rate was higher than other volume range (85-8 ml). CONCLUSIONS: In the balloon type infusor, the inside pressure of the balloon reservoir was not sustained evenly in the total reservoir volume range. But the infusion rate was parallel with the inside pressure of the reservoir and influenced by the reservoir volume. This may be owing to the change of the elasticity and tensile force of the reservoir according to the time and volume.