The report describes a study of the production and distribution of aggregates in Great Britain, in which the activities of producing, transporting and using aggregates are regarded as flows in the links of a network. The out-of-kilter algorithm is used to calculate the flow pattern which satisfies the demand for aggregates at least cost. The study considers the effects of rail depot density, rail capacity, the supply of sea-borne materials from coastal quarries and, to alimited extent, the effectss of some non-market costs on the transport of aggregates. Conclusions are drawn concerning the total costs of the distribution system, the planning of rail receiving depots for aggregates and the likely market for sea-borne materials