-From 1966-70 and 1972-74, we studied relationships between Ferruginous Hawk (Buteo regalis) reproduction and abundance of their main prey, the black-tailed jackrabbit (Lepus californicus) in the eastern Great Basin Desert of central Utah. Correlation tests showed significant relationships between jackrabbit abundance and hawk reproduction: the population of nesting hawks, number of nesting pairs, total eggs laid and total young fledged varied in synchrony with jackrabbit abundance. Key Factor Analysis and stepwise removal procedures indicated that jackrabbit abundance influenced hawk reproduction in the following order of decreasing importance: number of nesting pairs; failure of nesting pairs to achieve maximum clutch size; total number of young fledged and total number of young hatched. Regression tests revealed that two of these, the failure to nest and achieve maximum clutch size, are important limiting factors on reproduction and at least partially act to regulate the size of this Ferruginous Hawk population. Solomon (1949) proposed that predators respond to changing prey densities in two basic ways, functionally and numerically. In the functional response, a predator consumes prey of a given species at a rate according to the density of prey. In the numerical response, predator population density changes with changing prey density, usually through migration and/or corresponding adjustments in reproduction