We used repeated measures of castrated adult muskoxen during spring, autumn and winter to describe responses to a consistent diet. Increased feed intakes between spring and autumn were accompanied by increases in ruminal rates of in situ degradation for cellulose in hay and for protein in supplements. Microbial counts in ruminal fluid increased between spring and autumn but declined in winter. Total concentration of short chain fatty acid in the rumen also increased with feed intake but the proportions of individual acids varied between seasons. Microbial activity can change independently of diet quality in grazers from highly seasonal arctic regions.