Field and laboratory studies were conducted to develop a model of energy expenditure in a population of white-tailed jackrabbits (Lepus townsendii). Field work in southwestern Wyoming during 1985-87 showed that the breeding season commenced at snowmelt and ceased during late- July drought. Adult females reproduced relatively synchronously and produced a mean of three litters annually. Greatest fetal production occurred in the second litter period. Collections indicated a 1:1 sex ratio, few jackrabbits >2 years-old, and a density of 7 animals / km2 in the population. Postnatal growth was sigmoidal, culminating in heavier adult females than males. Using radio -telemetry, a circadian rhythm was detected in jackrabbit activity, with movement beginning after sunset and ending by sunrise. Observations showed that season, snow cover, weather, lunar phase, and predators influenced activity. Energetics studies established the pattern of seasonal acclimatization in the jackrabbit. Basal metabolic rate (BMR), pelage thickness, and body temperature increased but overall thermal conductance (C) and the lower critical temperature (LCT) declined from summer to winter. High winds and low air temperatures elevated metabolism interactively and their effects were most pronounced during summer. Metabolic rate dee lined with incident radiation at Ta< LCT during winter but not during summer. In newborn jackrabbits, body temperature dropped despite increased metabolism at Ta< 25°C. Cold tolerance and homeothermy developed with age. Based on these and published data, a FORTRAN model was written that simulated the energy expenditure of a population of jackrabbits. Metabolizable energy requirements for maintenance , thermoregulation, reproduction, growth, and activity were estimated. The model indicated that most energy (kJ·kg - l.day- 1) was required by adult females during lactation, adult males at the onset of breeding, and newborn juveniles. Energy expenditures for adult females, adult males, and juveniles were 191, 130, and 224 MJ·individual-1·km-2·yr-1, respectively. Total energy expenditure increased with wind and lower air temperature and decreased if juvenile huddling was simulated. The model indicated that the jackrabbit population is not limited by food. Estimated percent consumption of forage energy was 4%, assuming 50% of phytomass was edible, the population density = 100 jackrabbits/km2, metabolizable energy efficiency= 0.4, and the air was calm.