This paper describes the experimental use of a predictive automatic load-dispatching system, and discusses the experience gained. The objectives of control were the economic dispatch of active power only, while maintaining the security of the power system against loss of generation and transmission capacity. The control system used a digital computer, which allocated loads to individual generators for various times in the future, and transmitted instructions to the power stations, where they were automatically implemented by special machine controllers. The system was operational for about ten months. The aspects of control considered in the paper are prediction, the dynamic performance of the loading program, power-system security, frequency control, and operating economies. Other subjects treated in companion papers are the influence of machine-controller parameters on system stability, the performance of the machine controllers, and operator work load. Although it was only possible to examine the behaviour of a control system employing predictive techniques and regular instruction of generation, it was concluded that prediction of system conditions about 10min ahead was optimal for executive control of generation, but that regular instruction had serious drawbacks. Operating economies of a few tenths of one per cent seem feasible, but are markedly influenced by the way in which the constraints of the power system are quantified and handled.