We examined the effects of fasting and refeeding on body composition in 9 captive adult gray wolves, Canis lupus (6 males, 3 females), during May-June 1995. Body composition was estimated by the technique of tritiated water dilution. Wolves were immobilized and weighed, baseline blood samples were taken, tritiated water was injected, and additional blood samples were taken before fasting, after 10 d of fasting, and again after 2 d of refeeding. Male wolves lost 8% (P = 0.0001) and females lost 7% body mass (P = 0.01) during the 10 d. Males lost 54% of this mass in water, 28% in fat, and 18% in protein/ash; females lost 58% in water, 20% in fat, and 22% in protein/ash. Upon refeeding, male wolves consumed an average of 6.8 kg (15.3% body mass) of deer meat per day and females consumed 6.4 kg (18.7% body mass). All wolves regained their initial mass. Males regained 24% of this mass in water, 70% in fat, and 6% in protein/ash; females regained 35% in water, 51% in fat, and 14% in protein/ash. This study provided evidence that after prolonged fasting, captive wolves could quickly and efficiently regain lost body mass after refeeding.