A lot of studies have been performed on demographic changes in ungulate population due to density dependence, but little is done on density dependent changes in foraging behaviour which can be a mechanism of such demographic changes. This study presents results of density dependency and annual variation (linked to weather) on sheep grazing behaviour over three years and is the first study to experimentally test for density effects on lamb body mass. In a fully replicated, landscape-scale experiment on mountain pastures in southern Norway sheep were grazing at two density levels. Results of the analyses showed annual variation and density-dependence, but the latter always occurred in interaction with other variables.The use of the altitudinal gradient by sheep depended on density and year as well as prevailing weather. A clear sky and high temperatures made the animals to seek to higher elevations especially when resting. This might be an adaptation to avoid insect harassment or to keep an optimal body temperature. There was no effect of density on the selection of vegetation types. Sheep selected meadow first both in 2003 and 2004, while ranking of other vegetation types differed between years. The comparison between low quality (Deschampsia flexuosa) and high quality (herbs) forage showed effects of density, year and age. The use of D. flexuosa varied slightly between years and due to density, while there was a marked effect of age. Ewes ate more grass than lambs did and sheep at high density increased their intake of low quality forage over the season likely due to higher competition. Lambs ate more herbs than ewes in 2003 and 2004 than in 2002 and sheep at low density were able to utilize herbs for a longer time throughout the season. One main result was that lamb body mass showed increased density-dependence from 2002 to 2004. Further, the difference in mass between triplet lambs and singleton lambs was more pronounced at high than at low density. Since triplet lambs growth rates are more sensitive to population density, this suggests monitoring their mass can be used for managing sheep densities on mountain pastures in Norway.