The relative importance of two organochlorine pesticides in the recent reproductive failure of raptors was investigated. Captive barn owls were fed 3.0 ppm DDE and 0.5 ppm dieldrin; doses were given separately and in combination for two years. Breeding success was followed from the laying of eggs through natural incubation and rearing of young. DDE was associated with significant eggshell thinning, egg breakage, embryo mortality, and reduced production per pair. Dieldrin alone was associated with slight but significant eggshell thinning, but not with reduction of breeding success. Ecological implications of the results are discussed; it is suggested that DDE had a much more severe effect on reproduction in wild raptors than dieldrin, which contributed to their decline primarily through adult mortality.