The study was conducted to examine the importance of forage resources in limiting peak population density of black-tailed jackrabbits (Lepus californicus). The research design combined field studies of jackrabbit diet and vegetation impact with computer simulation of herbivore nutrition and population dynamics. The relationships between available forage, dietary intake, energy and protein balance and population changes were compared for high and low population density. Hypotheses tested were : (1) Dietary composition is unaffected by availability above 10 kg/ha; (2) Jackrabbits do not require more forage than is available at high population densities; (3) Dietary composition is not significantly different at high and low densities; (4) Dietary composition is not significantly different between sexes; (5) Nutritional balance of lactating females is not significantly altered at high density; and (6) Nutritional balance of females entering the breeding season is not significantly altered at high density. The field studies were designed to measure dietary preference by the jackrabbits, the availability of herbage, and utilization of selected plant species. Jackrabbits ate nearly all forage types available but primarily consumed dominant shrubs in fall and winter, suffrute scents in fall and winter, grasses in spring and summer, and forbs in summer. There was no significant difference between dietary composition or preference rating among density periods or sexes. Dietary percentage of preferred forage species was directly related to availability above 10 kg/ha. Jackrabbits removed an average of 30 to 40 percent of individual plants of Kochia americana, but only browsed on 5 toll percent of all available plants. Total removal of Kochiastanding crop averaged only 3.7 percent. The model used in the simulation studies incorporated relationships involving existence, activity and production (growth, gestation and lactation) requirements to estimate forage requirements. Intake and requirements affect energy and protein balance which are related to body weight changes and production. Model mortality rates are modified by nutritional deficiency as a fraction of the requirements. The following conclusions resulted from model simulations. Forage requirements are less than 1 percent of available forage, even at high population density. Nutritional balance of lactating females was unaffected by observed diets or population density. Nutritional balance of females entering the breeding season is unaffected by diet but may be affected by early breeding date or large litter sfze. Individual nutritional response and its interaction with food does not cause significant change in population mortality rates. Nutritional deficiency may contribute to mortality, particularly during lactation or winter periods, but these effects are independent of population density.