This report presents a joint Nordic project on method development for measurement of release rate of active substances from antifouling products. The aim was to evaluate whether a prolongation of a standard method for release rate measurement of copper from antifouling paints may give information which is of importance for the evaluation of the environmental effects of modern antifouling paints. The background was the suspicion that some new antifouling paints for pleasure boats have non traditional release behaviour in that the release rate is low at start and then increases to a higher level. To date, the standard methods do not detect this kind of release pattern, and consequently, risks for the aquatic environment due to high release rates later in the service life of the paints are not properly evaluated. In the report, a prolongation from the standard 45 days to 122 days is studied, and some conclusions are drawn regarding the additional information for environmental risk assessment that it may give. The project was conducted by Åke Forsberg, AB Thalassa (test laboratory) and Kurt Haglund, Swedish Chemicals Agency and supervised by the Nordic Biocides Group under the Nordic Chemicals Group. The project was financially supported by the Nordic Council of Ministers.