Muskoxen (<i>Ovibos moschatus</i>) are relatively large herbivores living in an arctic environment where forage is sparse and its availability during long winters is often much reduced by snow and ice. Muskoxen cope by obtaining energy and nutrients from the low-quality sedges and grasses they eat, and from body reserves accumulated in summer and autumn. The objectives of this study were to evaluate the physiological and energetic adaptation of muskoxen to digestion of graminoid forage, and to measure the extent and reproductive significance of seasonal changes in body composition of female muskoxen. The first objective was addressed using two studies with captive muskoxen in Saskatoon, and the second during a field study on Victoria Island, Northwest Territories. In the first study, changes in body weight, intake, retention time and digestibility of a supplemented brome-alfalfa hay were measured in mature muskoxen during two seasonal cycles. Mean daily dry matter intake of breeding females was highest in July and August (62 ± 3.6 g/kg^0.75) then declined slowly to its lowest point in March and April (41 ± 0.7 g/kg^0.75). Mean retention time of hay, calculated from a chromium marker, declined (P<0.01) from 114 ± 4 h in March to 95 ± 4 h in July. Apparent organic matter digestibility decreased (P<0.01) from 74.7 ± 0.8% in winter to 61.7 ± 1.3% in summer. Compared with other ruminants, muskoxen are grazers exceptionally well-suited to slow, thorough digestion of graminoid forage. In the second study, the voluntary intake and apparent digestibility of a low-protein grass hay, similar in composition to diets of wild muskoxen in winter, were measured in five mature muskoxen and three mature Hereford cows during late winter 1993. Both species maintained body weight on the experimental diet but daily dry matter intake of the muskoxen was only 1/3 that of the cattle (32 ± 2 vs. 96 ± 3 g/kg^0.75; P<0.001). Apparent digestibility of organic matter, measured using chromic oxide, was higher in the muskoxen than in the cattle (58 ± 2 [SE] vs. 50 ± 3%; P=0.03). Muskoxen are particularly well adapted to digesting low quality graminoid forage at exceptionally low rates of intake. In the third study, the anatomical and chemical body composition of 22 muskoxen that varied widely in size, age and condition were analysed, and measures of body weight and condition were evaluated as predictors of body composition. The leanest muskoxen were four 5 to 15-day-old calves with 3.5 ± 0.5% (mean ± SE) fat in the IFBW, and the fattest were three females with 24.8 ± 1.4% fat. Muscle weight was best predicted from weights of individual muscles, protein weight from IFBW, bone weight from the weights of individual limb bones, and ash weight from IFBW. Combining kidney fat weight and a measure of body weight with up to three other measurements allowed dissectible and total fat weights to be estimated accurately. The extent of fattening in muskoxen exceeded that found in other wild ruminants with the exception of Svalbard reindeer (<i>Rangifer tarandus platyrhynchus</i>). In the fourth study, the body weight, body composition and reproductive status of 202 muskox cows from Victoria Island were analysed during 16 seasonal collections from 1989 to 1993. Adult muskox cows were lightest and leanest in May, when the ingesta-free body weight (IFBW) of newly-lactating adult cows was 116 ± 2 kg (adjusted mean ± SE) and a similar 112 ± 3 kg in nonlactating cows. Thereafter, the IFBW of lactating cows was virtually unchanged until July, increased rapidly during August, peaked in September at 166 ± 4 kg, then decreased to 148 ± 2 kg in November. In contrast, the IFBW of nonlactating cows increased throughout summer, reached a higher peak in September (176 ± 7 kg; P < 0.01) and changed little by November (164 + 2.1 kg). Pregnancy rates in this population increased from 0 in females 1.5 years old in the autumn to 25% in 2.5 year-olds and 62.5% in cows at least 3.5 years old. Lactation did not appear to peak until at least two months after calving, and was sometimes extended to more than a year. Lactation in November did not preclude pregnancy, but pregnant females at this time were fatter and heavier than nonpregnant cows. These results indicate substantial flexibility in reproductive patterns of muskoxen and an unique ability to maintain condition through long arctic winters. Based on these studies, the persistence of muskoxen through arctic winters can be ascribed in large part to exceptionally low maintenance requirements, along with an ability to thoroughly digest low-quality graminoid forage. Female muskoxen also deposit large quantities of fat in late summer and are able to conserve a large portion of them through early and mid- winter, and use them to support late-winter lactation. Muskox reproductive rates are strongly sensitive to nutritional influences. This suite of K-selected characters in muskoxen is consistent with a slow metabolism and a low rate of population growth.