In many control problems, e.g. in biology, time delays of the order of seconds or more play an important role. The present device is intended for simulations of such problems. Its accuracy (0·1 per cent) is of the same magnitude as that for most analogue computer components. It has been used in preliminary experiments (Fig. 5) to study biological rhythms. However, many differential equations with time delay, including those with more complicated weighting functions than that in Fig. 5, can be successfully studied using this method.