The paper assesses the performance of the frequency-control loops of the machine controllers installed in the power stations participating in the automatic-load-scheduling experiment. It first describes the role of these control loops and explains the mechanism of frequency control, and then examines in detail the stability and performance of the machine-controller frequency loops. Included are results of extensive field tests carried out on both individual machines and isolated systems and of analogue-computer simulations. Stability and performance are shown to be closely dependent on the response characteristics of the conventional speed governors, as well as on the controller parameters.