A study is made of the transient motion of a suddenly released column of liquid poison, such as mercury or borated water, as it moves through the control tubes of a reactor. Two basic systems are described. The first system consists of a tank of liquid poison, situated above the reactor, connecting directly to control tubes extending down into the core. The second system is a U-tube arrangement in which the liquid poison enters through tubes that extend upward from the base of the core. By making the tank or standpipe diameter larger than the control tube diameter it is possible to obtain initial fluid column accelerations greater than gravitational acceleration. When solid control rods are used in reactors, it is necessary to employ auxiliary springs to give an initial acceleration greater than gravitational. High initial accelerations are inherent features of liquid poison scram systems. It is found that the initial acceleration is gravitational acceleration multiplied by the ratio of standpipe area to control tube area. An exploitable feature of the U-tube system is that the liquid column undergoes a natural rapid deceleration. It may therefore be possible to do away with damping devices that are often required at the base of a solid control rod channel. The equations of motions of the system are derived and integrated. The general solution in both cases gives the displacement time explicitly in terms of elliptic functions; in the U-tube system the unconfined fluid column is found to execute uniform elliptic oscillations.