We measured fluctuation of injection pressure by several types of commercially available syringe pumps in order to investigate the effect of syringe volumes (Terumo syringe) and speed settings on the irregularity of injection speeds. We recorded the injection pressure continuously with various speed settings, except that one of the pumps injected irregularly at the settings of 2 and 1 ml.h-1. With 50 ml syringes, only two of the six pumps injected precisely at all the speed settings. With the other four types, a steep increase in pressure was recorded at the speed setting of 1 ml.h-1. However irregular infusion was never observed by using a syringe of a high resistant type (TOP syringe). With 30 ml syringes, injection pressures fluctuated in most of the six syringe pumps at a speed lower than 5 ml.h-1. In conclusion, changes in injection pressures depend on types of syringes, syringe volumes and speed settings. We urge not to use a 30 ml syringe to infuse vasoactive drugs with syringe pumps.