ABSTRACTThe conference of Ministers representing North Sea littoral states, held in November 1987, agreed certain initiatives to reduce inputs of potentially dangerous substances to the North Sea from land‐based sources. The UK Government has responded by proposing reductions of substances on a ‘red list’, selected on the basis of ecotoxicity, persistence, potential for bio‐accumulation and estimated concentration in the environment.This paper describes the approaches which have been used in selecting candidate substances for the red list and other priority‐setting exercises, highlighting the difficulties presented by the lack of data for individual chemicals and the use of default values.The estimation of input loads via rivers and direct discharges is discussed, with particular reference to monitoring requirements and analytical performance