A grazing experiment with sheep was carried out in 1990-1993 on natural, semi-natural and cultivated pasture on clay soil. The pastures were located in Southern Finland and were moderately contaminated with 137 Cs by Chernobyl fallout. Natural pasture refers to forest pasture and serai-natural pasture to set-aside field pasture, the latter having been under cultivation about 15 years ago. The transfer of 137Cs to sheep was clearly higher from forest pasture than from the other two pastures and it was lowest from cultivated pasture. The transfer was higher to muscle and kidney than to liver and heart. The transfer of 137Cs to plants and to meat varied with years. Seasonal variation in the plant 137 Cs was followed-up on forest and set-aside field pasture; the activity concentration of plants reached a maximum in June, a lesser increase occurred later in the autumn. In 1993, which was considered an average year with respect to 137Cs transfer to plants, the mean soil-plant transfer factors of 137Cs for forest, set-aside field and cultivated pastures were 1.78, 0.36 and 0.09, and soil-meat aggregated transfer factors 11.0, 0.28 and 0.03, respectively.