Plastic pools (surface 6.6 m/sup 2/, volume 4.2 m/sup 3/) were equipped with a flow-through system providing 2.51 min/sup -1/. Except for fish predators the main components of the flora and fauna of the Baltic littoral zone were introduced into the pools to form a model of the ecosystem. During 8 weeks the macroscopic epifauna and infauna of the bladder wrack Fucus vesiculosus L. were found to be qualitatively and quantitatively fairly stable, and the number of aerobic heterotrophic bacteria showed little variation. Oxygen concentration, temperature and pH were recorded and compared with values measured in the littoral zone. The results indicate good agreement between the characters of the model system and of the natural littoral ecosystem. This together with the observed stability and the possibilities for controlling and measuring the conditions in the system makes us believe that the model is a valuable tool for assessing toxic effects on the littoral ecosystem.