A single-shot rapid-mixing device is described for the rapid addition of solutions of radiation-modifying agents, to cell suspensions, at well-defined times relative to a pulse of radiation. The liquid injection system could be used to initiate or quench a wide range of chemical or biochemical reactions. The rapid-mixing device is based on a syringe driven by a stepper motor and can inject up to 2 cm3 liquid in less than 100 ms. The radiation source, a 4 MV Van de Graaff accelerator, provides an electron beam which is deflected from the beam dump on to the sample in two stages, providing a 10 ms radiation pulse. A digital delay circuit defines the interval between mixing and irradiation. The apparatus has been designed to study the kinetics of processes that occur over a time range extending from about 0.1 s to some minutes. It bridges the gap between the ranges available with conventional fast-mixing and those using standard X- or gamma-irradiation methods. The time resolution of the technique has been examined by following the timecourse of radiosensitization by oxygen in mammalian cells. The timecourse of radioprotection of aerobic mammalian cells by dithiothreitol has been measured using the technique.