Foods and predatoFprey relationships were ascertained from analyses of 4,651 pellet and stomach samples of great horned owls (Bubo virginianus) from the western prairie region of Missouri during the 20-year period, 1949 to 1968. Additional data were obtained from 70{) pellets of barred owls ( Strix varia ), 419 pellets of screech owls ( Otus asio ), 180 pellets of long-eared owls ( Asio otus ), and 513 pellets of red-tailed hawks (Buteo jamaicensis). Cottontail rabbits (Sylvilagus floridunus) and small rodents (mice and rats) were staple foods for all species. Other mammals and birds occasionally were taken in important amounts. Predator utilization of rabbits related poorly to abundance, as shown by midsummer rabbit census trends. Consumption of small rodents varied greatly from year to year. Per- centages of meadow mice (Microtus ochrogaster) and white-footed mice (Peromyscus spp.) in yearly diets showed little evidence of regularity in peak populations. Utilization of cotton rats (Sigmodon hispidus texianus) was strongly suggestive of irruptive peak populations at 8-year intervals, 1951 1959, and 1967, followed by rapid declines to very low levels. Foods of avian predators probably wili not serve as accurate measures of average prey populations, but unusually high populations of small rodents can be detected through long-term studies of food habits.