Eight concrete land-based mesocosms have been set up for a study of the effect of nutrient enrichment on littoral hard bottom ecosystems. The construction of the mesocosms and the community establishment were initiated 2 yr ahead of the MAST-III project. The littoral communities were established by transplanting rocks with attached macroalgae and associated animals from the Oslofjord, followed by a period of natural community development which has resulted in a highly diverse flora and fauna. During this 2-yr pilot project, the efficiency of relevant techniques and statistical design for the EULIT experiments in the mesocosms have been tested, as well as the performance of the mesocosms ahead of the nutrient manipulation. The pilot project has thus extended the total experimental period. The mesocosms are fed with water from 1 m depth in the Oslofjord - residence time about 2 h - which also acts as a source for spores, zygotes and larvae. A tidal regime with a 36 cm amplitude is maintained and waves are regularly generated. Oxygen concentration, temperature and salinity are measured continuously by probes. Nutrient addition started in May 1998. Nutrient and carbon input and outflow are measured on an average weekly basis. Community analyses, species compositions, biomass, growth and grazing of the different components of the mesocosms are studied during 3-week periods (spring, summer and autumn) each year.