To assist in the study and understanding of plant dynamics, circuit interactions, mill operational behaviour and automatic control schemes, a model of the dynamic behaviour of autogenous (ag) and semi-autogenous mills (sag) is currently under development at the JKMRC. The model is being developed to provide an accurate dynamic response in terms of power draw, grinding charge level, slurry level and product size distribution for changes in feedrate, feed size, feed hardness and water addition. The conceptual structure of the model is described together with dynamic data from 2 full scale high aspect ratio autogenous mills which will be used to validate the model. The data were obtained from a series of step tests and comprise trends in power draw bearing pressure, solids and water flowrates as well as an indication of the fraction of coarse rocks in the feed. This latter quantity was obtained from a novel, yet simple device for counting rocks which will be described. The mills measured response to these step changes is presented together with an explanation for their behaviour in terms of the physical changes which took place inside the mill.