This study aimed to investigate the bone development of two mountain sheep breeds during natural summer grazing either in the lowlands or on different characteristic alpine pastures. Pasture types differed in topographic slope, plant species composition, general nutritional feeding value, Ca and P content, and Ca:P ratio of herbage. Twenty-seven Engadine sheep (ES) lambs and 27 Valaisian Black Nose sheep (VS) lambs were divided into four groups of 6 to 7 animals per breed and allocated to three contrasting alpine pasture types and one lowland pasture type. The lambs were slaughtered after 9 wk of experimental grazing. The steep alpine pastures in combination with a high (4.8) to very high (13.6) Ca:P ratio in the forage decreased total bone mineral content as measured in the middle of the left metatarsus of the lambs from both breeds, and cortical bone mineral content and cortical bone mineral density of ES lambs. Breed × pasture type interactions occurred in the development of total and cortical bone mineral content, and in cortical thickness, indicating that bone metabolism of different genotypes obviously profited differently from the varying conditions. An altitude effect occurred for 25-hydroxyvitamin D with notably higher serum concentrations on the three alpine sites, and a breed effect led to higher concentrations for ES than VS. Despite a high variance, there were pasture-type effects on serum markers of bone formation and resorption.