An investigation into the decline of blue wildebeest numbers at Etosha National Park was made during the period 1974-1978. Aspects of wildebeest ecology which were investigated were activity patterns, energy and protein budgets, nutrition, nutritional status, disease and parasites, predators and scavengers, and population structure. Activity patterns in the free-ranging population were measured over a period of one year, using marked individuals and by scanning herds. The population spent, on average, 53% of its time resting, 33% grazing, 12% in movement, 1,5% in overt social encounter sand less than 0,5% drinking and suckling. Photoperiod and temperature were the primary correlates of activity/inactivity and a linear relationship existed between increasing temperature and increasing inactivity. The activity data were used to calculate an energy budget for maintenance and activity in the population, while energy demand for growth, gestation, lactation and homeothermy were estimated by formulae based on domestic ruminants. Mean increment over resting metabolic rate for free existence in wildebeest approximated 2,0. This represented a mean annual energy demand of 8,5 gigajoules per wildebeest. Similarly, a protein budget was estimated for the population on a seasonal basis.